Bad 2 Bad: Apocalypse is a fun little game, but the lack of actual information bothered me. I eventually let my curiosity get the better of me and started trying to figure out how everything in the game worked. Because there isn't much solid information online, I decided to share my findings here and hopefully help other people who were confused as well.

Note that I didn't look through the code and my investigations/experiments weren't very scientific. Some information I never figured out, though the majority I'm fairly confident on. If you discover anything I missed, please feel free to let me know in the comments.

So what do all the stats do?

Important Note:
Unless specifically noted by [Squad] being listed before the stat name, skills only affect the character that has them. Getting Recovery on Isabella will only affect Isabella. Getting Reload Speed on Dail will only affect Dail. Of special note, Mobility only does something for the current Squad Leader. Only take Mobility gear or skills on your Squad Leader. Other characters having +Mobility won't make your squad move faster.

• Damage
  How much damage the weapon does per shot. 150 damage means 150 damage per bullet + any +% damage bonuses from ammo, skills, and proficiency. They don't list the actual final number.
  
  Of note, the shotgun's damage works differently because it fires six "pellets" per shot; I'm not sure on how the damage is split up, but it doesn't do the listed damage per "pellet". Best guess, it takes your total damage and divides it by the number of "pellets".
  
  
• Burst Speed
  How fast the weapon fires. Fire Rate bonuses from skills and proficiency affect this by lowering the time between shots. I'm not sure on the exact timings by stat, but a higher value means faster shots.
  
  
• Control
  How much the aiming reticle expands or blooms out when firing, how fast it returns back to neutral after firing, and what the weapon's neutral or "full/best" accuracy is. Basically, how accurate your weapon is and how well it can handle recoil.
  
  Sprinting and movement affect this; standing still will give you your best possible accuracy, with walking and sprinting giving you progressively worse accuracy.
  
  
• Range
  How far your shots travel before disappearing. There doesn't seem to be damage fall-off; shots either hit and do damage, miss, or reach the end of their Range and are deleted.
  
  350-ish Range seems to be about half the screen width on Basic Formation, though how far out the screen pulls is based on your Formation and if you're Sprinting or not, so that number is just a best guess.
  
  
• Reload Speed
  How fast you reload. Reload Speed doesn't seem to be listed anywhere other than some skills and attachments, so I'm not certain on exact times, but while it is different by weapon type, I don't think it's based on how much ammo is in a weapon. Basically, putting in a bigger magazine doesn't seem to make you reload slower; actually, since magazines have +Reload Speed% on them, you get more ammo and you reload faster than normal.
  
  
• Mobility
  How fast your squad moves. Even small amounts like +2% can feel pretty dramatic. This can be a confusing stat, so I'm going to go into it a bit.
  
  Mobility is only counted for the squad leader. Skills and equipment on other characters do -not- affect squad movement speed. This includes the skills Lightweight and Isabella's Mobility Specialization; they will only have an affect if that character is squad leader. Basically, your squad leader should stack gear with Mobility and preferably have a Mobility skill learned, and everyone else should avoid the stat entirely.
  
  Mobility is affected by the various stances.
  
  
• Firing Mobility
  Not certain. This is not the same as Mobility. It doesn't increase your movement speed and doesn't help reduce your reticle bloom while moving.
  
  
• Stopping Power
  How much your shots push back enemies. It seems like more damage increases push back, so a Sniper Rifle will shove an enemy back more per shot than a Machine Gun will, but a Machine Gun fires a lot more frequently than a Sniper Rifle, so it feels like they mostly break even. It can be a very useful stat against Wilders.
  
  
• Recovery Rate
  How effective healing items are on you. It isn't a passive heal-over-time. It seems to be a straight additive to any healing received; so someone with +50% Recovery Rate who uses an item that heals 100 HP will instead heal 150 HP.
  
  
• Defense/Received Damage
  Not certain. Obviously Defense reduces damage taken in some way, but I'm not sure what kind of math is involved entirely because Received Damage exists as a modifier and they both seem to be different things.
  
  Received Damage seems to be a simple reduction in damage taken, so DR -10% vs. 300 damage taken = 270 damage taken. Defense, I have no idea. The percentage symbol may be erroneous, and it may be a flat amount reduced from each attack (Defense 10.0 vs. 300 damage = 290 damage taken), but I honestly don't know.
  
  
• Headshot Defense
  Not certain. As far as I can tell, headshots work like critical hits, doing more damage than normal body shots. Headshot Defense either has a percentage chance to negate that effect, or more likely reduces the extra damage from the headshot by the percentage listed. Dail can go over 100% Headshot Defense, so I'm not sure if it just caps at 100% with overflow wasted or what, but that is my current belief.
  
  
• Status Resistance
  Not certain. This affects things like Sneaker poison, but I'm not sure how. It either is a percentage chance to negate the effect or a percentage reduction in how much the Status Effect affects you; i.e., with Sneaker venom, how much damage you take from it, so 10% SR vs. 100 poison damage would = 90 damage total. It may instead affect damage by tick, but I'm not sure.
  
  
• Aggro
  Not certain. As far as I can tell, if given a choice between targets enemies will aim for or attack in melee the character with higher/highest Aggro. I have no idea on specifics. From casual observation however, enemies do tend to prefer targeting characters with a high Aggro stat compared to characters with a baseline or lowered Aggro stat.
  
  
• Additional Damage to SD
  How much extra damage you deal against Shields.
  
  
• Headshot Damage
  Not certain. It's either additive or multiplicative, no idea which. It does the basics of what you assume though - it increases the damage you deal when hitting an enemy's head.
  
  
• Penetration
  How many enemies a shot will travel through and damage before stopping. Shots without Penetration will strike one target and vanish; shots with Penetration will damage the target, the pass through and potentially damage more targets along its travel path equal to the Penetration value.
  
  Basically, if a bunch of enemies are in a line, a high Penetration value lets you hit all of them with one shot by punching through or over-penetrating.
  
  
• Block Point
  How many additional BP your equipped Shield has. If you use a Shield with 100 BP, then +50 Block Points means it now has a maximum of 150 BP. BP is the HP of your Shield; if it hits zero, you get stunned for a moment and drop it on the ground (it is not destroyed, you can pick it back up.)
  
  
• Block Point Recovery
  How many additional BP your Shield recovers per second. 5.00 BPR = 5 BP/sec recovered.
  
  
• Recovers Block Points on 2nd Weapon Battle
  Quite a name there. This stat gives the Shield user back BP when they use the weapon in their second slot.
  
  When your Shield drops to zero BP you drop it. Even when you pick it back up it remains at zero/very low BP, which means in difficult fights it almost instantly gets dropped again. This seems to be meant as a way of stopping that situation by allowing you to regenerate your Shield's BP while using your other weapon.
  
  Example: Dail's Shield is brought down to 10 BP. Not wanting to have it drop, you swap to his second weapon, a Machine Gun, and fight with that for a while. As you do, the blue bar under his red HP bar - his BP bar - goes up. Once it's full enough, you swap back to your Shield.
  
  
• Bonus Ammo Chance/Free Ammo Chance
  Not certain. Free Ammo Chance I believe is a percentage chance to not use up special ammo when firing it. Bonus Ammo Chance, I have no idea.
  
  
• Wilder Attack Specialization
  How much extra damage you do to Wilders.
  
  
• Humal Attack Specialization
  How much extra damage you do to Humals. This works on Pillagers, but I'm not certain if it works on Apex.

What's a good squad? There's plenty of options (and even more if you get the character DLC), but I'll list a good generalist squad here and explain why we're taking each character and how to build them.

• Positions
  Bagheera (Squad Lead)
  Dail (Second Slot)
  Beo and Isabella (Third and Fourth Slots)
  
  Bagheera
  Bagheera is our Squad Leader instead of Isabella for several reasons. She has the Lightweight skill, making the squad move 12% faster at the cost of a few inventory slots, while Isabella has Mobility Specialization for +10% but no downside. The lost inventory slots aren't honestly noticeable once everyone has a backpack, and Isabella is a bit starved for Skill points while Bagheera can pick up everything she wants comfortably, so we'd rather spend the points for Mobility on Bagheera. Bagheera also has some defensive skills in her list, which is important since if the Squad Lead dies you either have to resuscitate them or the entire squad falls.
  
  Dail
  Dail is in the second slot because he is our tank, and several stances place the second slot character out front to intentionally absorb damage. He'll be using a Shield/Pistol in his primary weapon slot and a Machine Gun in his secondary weapon slot. Once he learns SD Enhancement II he should use his Shield/Pistol combo until his BP gets low, then swap to his Machine Gun until his BP gets back up again. In general you want the shield out against Humals/Apex, but feel free to use the Machine Gun against Wilders if you prefer the extra damage and push back against them.
  
  Isabella/Beo
  Isabella and Beo go into the third and fourth squad slot; which one goes where doesn't really matter too much, though one of the formations puts slot four in the very back so you may want to set Beo there due to his longer range. That said, again, it isn't really that important.
  
  
• Skills
  
  Note: Any skills leveled to less than maximum (i.e., 3/5, 0/5) will be listed in bold for easier visibility. Also, you get a total of 50 Skill Points to spend when fully leveled.
  
  Bagheera
  Tier 1
  o Shotgun Fire Rate Enhancement 5/5
  o Stopping Power Enhancement 3/3
  o Strength Enhancement 10/10
  
  Tier 2
  o Shotgun Range Enhancement 4/4
  o Lightweight 3/3
  o Defense Specialization 5/5
  
  Tier 3
  o SD Attack Specialization 5/5
  o Fire Mode 3/5
  o Resistance Enhancement 3/3
  
  Tier 4
  o Shotgun Damage Enhancement 5/5
  o Enhance Recovery Rate 0/5
  o Defense Enhancement 5/5
  
  Dail
  Tier 1
  o Machine Gun Precision Fire Specialization 5/5
  o Reload Specialization 3/3
  o Hardening 10/10
  
  Tier 2
  o SD Enhancement I 3/3
  o Control Enhancement 5/5
  o Stonehead 5/5
  
  Tier 3
  o SD Enhancement II 3/3
  o Firing Mobility Specialization 0/5
  o Aggro Increase 3/3
  
  Tier 4
  o Machine Gun Fire Rate Enhancement 5/5
  o Porter 3/3
  o Beastify 5/5
  
  Isabella
  Tier 1
  o Submachine Gun Damage Enhancement 5/5
  o Stopping Power Enhancement 3/3
  o Strength Booster 10/10
  
  Tier 2
  o Humal Attack Specialization 5/5
  o Fire Mode 5/5
  o Scouting Ability Enhancement 2/2
  
  Tier 3
  o Control Mode 5/5
  o Grenade Enhancement 3/3
  o Mobility Specialization 0/3
  
  Tier 4
  o Damage Booster 5/5
  o Recovery Specialization 2/5
  o Defense Enhancement 5/5
  
  Beo
  Tier 1
  o Widow Maker 5/5
  o Penetration Specialization 3/3
  o Survival Enhancement 10/10
  
  Tier 2
  o Battle Rifle Precision Fire Specialization 5/5
  o Battle Rifle Range Enhancement 4/4
  o Frugality 0/5
  
  Tier 3
  o Lightweight 0/3
  o SD Attack Specialization 5/5
  o Material Collecting Specialization 5/5
  
  Tier 4
  o Ammo Selection 5/5
  o Grenades Owned 3/3
  o Decrease Received Damage 5/5
  
  
• Equipment
  
  For every character, the final item in a Generation of equipment (white, green, blue / Generation 1, Gen 2, Gen 3, etc) is going to be the best choice, it just depends on if you're willing to wait until you get 8 of the required parts or you want an early jump from one Generation to another by taking the 3 part option. In general it's best to just use guns you find until you get 8 weapon parts for the Generation you're on, then make the best item you can in that Generation, as the difference is pretty big, but going from Gen 2 (Green) to Gen 3 (Blue) generally shifts a lot of weapons towards having full attachment slots, so it can be worth making early weapons.
  
  An example of an exception is Beo's Battle Rifles; it's worth taking the BR-15 (the earliest Gen 3/Blue Battle Rifle, the one that takes 3 parts), because it has full attachment options and, most importantly, a slot for an underslung grenade launcher.
  
  o Bagheera
  Shotgun. For armor equip her with anything that has Mobility on it as a priority over any other stat. Second best is anything that lowers Aggro or increases her weapon stats, like Range or Fire Rate.
  
  o Dail
  Shield/Pistol with a Machine Gun as his secondary weapon. For armor, get the most Defense and Headshot Defense you can. Equipment that increases his Aggro is also a priority. Secondary stats are anything that helps him take more damage or live longer, or barring that, extra damage stats.
  
  o Isabella
  Submachine Gun. For armor, lowered Aggro and weapon damage stats.
  
  o Beo
  Battle Rifle. For armor, lowered Aggro and weapon damage stats.
  
  
• Usage
  
  So why these characters, skills, and equipment?
  
  All four have good ultimate abilities. Bagheera's Flashbang can stun one or two enemies (the radius is surprisingly small), and while it can't stun bosses it's still a great emergency button. Dail can massively increase his damage to machines and shields, some of the more annoying/dangerous enemies you encounter. Isabella can heal everyone to full nearly instantly for the price of four Bandages. And Beo gives everyone -25% Received Damage for 20 seconds, which is a pretty big deal in nasty fights.
  
  Bagheera is a solid damage dealer with some defense, a Mobility skill, and the Stopping Power Enhancement skill. Her Cooldown Ability is also very good. This plus how starved for Skill Points Isabella is makes her a good choice as Squad Leader.
  
  Dail is our tank. The difference between using a Shield tank and not is massive against Humals and Apex. Against Wilders we can swap to his Machine Gun for extra damage and push back.
  
  Isabella is our healer and damage dealer. She, like Bagheera, gets the Stopping Power Enhancement skill, which helps keep Wilder enemies back and away from the squad, reducing overall damage taken. Her Cooldown Ability is extremely helpful in stressful fights. While keeping a lot of food and medical kits around can negate the need for her Ability, her other bonuses (minimap exploration and grenade damage) make her useful no matter what.
  
  Beo is our dedicated damage dealer. He gets extra damage, headshot damage, and access to the underslung grenade launcher, which does a ton of damage on a short cooldown without using any actual grenades. His Ability is also a very strong defensive tool for the squad during bad fights.
  
  For skills, all four can take pretty much everything actually worth taking in their Skill Trees without any real issues. The only ones that would like more Skill Points is Isabella to finish Recovery Specialist and possibly Beo for Frugality. Everyone else gets everything they want/need.
  
  For equipment, the choices are fairly simple - take whatever they're specialized in. You could use four Assault Rifles/Battle Rifles and go for Grenade Launcher spam and be totally fine, but it feels more fun to me to build for what they're intended to use. Having Bagheera and Isabella with Assault Rifles in their secondary weapon slot entirely for Grenade Launcher usage isn't a bad idea though. (Note: Multiple Grenade Launchers just reduce the cooldown of the ability, they don't let you rapid-fire grenades).

There's still quite a lot out there to investigate and understand, but I feel like this is a pretty good list explaining the questions I had myself when I was playing, and basic questions I feel other players will likely have as they play through the game as well.