This is just a simple guide I'm throwing together really quick to be helpful for players who are new to this game and not sure what the items do.

Crystal Vial
Blue vial that gives you 10% health instantly when you run into them.

Flechette
A green potion.
As fighter: A 3 second grenade that will bounce a bit along the ground and then explode.
As cleric: It drops a green poison cloud where you're standing. Make sure to move after using this, ideally backpedal. It's actually great for doing tons of damage to groups of enemies in a choke point.
As mage: Drops where you're standing and then explodes after a second so backpedal for this one too.

Quartz Flask
Big purple potion. Heals you for 25% of your health.

Blue Mana
Looks sort of like a small upside down pyramid. Restores your blue mana which can be used for weapons such as your Serpent Staff.

Chaos Device
Looks like a round disc with a red plus sign on it. Sends you back to the start of a level. Useful for escaping a bad situation (though not as useful as being a quick loading save scummer like me).

Mystic Urn
Looks like an urn with an ankh on it. Restores you to full health. The ankh is the symbol of life after all!

Amulet of Warding
Blue amulet with a strap that hangs from it, because items all float in these classic FPS games.
As fighter: 1 point of Armor Class (or AC)
As cleric: 4 points of Armor Class
As mage: 5 points of Armor Class

Banishment Device
Looks like a red plus sign. Banishes enemies away from you and into some other area in the level. Does not work against Death Wyvern, Heresiarch, or Korax, who are bosses.

Disc of Repulsion
Looks like a disc made of gold rings. Stacks up to 25. When you use it, it repels enemies away and can deflect enemy projectiles. Repelled enemies that hit anything, including walls or each other, will take damage. This can also blow enemies off cliffs to their deaths.

Boots of Speed
Temporarily doubles your movement speed when used.

Dark Servant
Looks like a maulotaur doll. When you use it, it throws it on the ground where it becomes a friendly maulotaul that will help you. The maulotauls are big horned bovine enemies, not to be mistaken for the 2 headed ordinary enemies you see in most of the game, those are called Ettins.

Dragonskin Bracers
Looks like a pair of bracelets with a purple glow around them. When used, temporarily gives you 4 points of Armor Class.

Falcon Shield
As fighter: 4 points of Armor Class
As cleric: 5 points of Armor Class
As mage: 3 points of Armor Class

Icon of the Defender
Looks like a small gray castle tower item.
As fighter: Makes you invulnerable.
As cleric: Makes you partially to totally invisible, when invisible enemy projectiles will pass through you.
As mage: Deflects enemy projectiles.

Krater of Might
Looks like a chalice with purple juice in it. When you use it, it fully restores all blue mana and green mana.

Mesh Armor
As fighter: 5 points of Armor Class
As cleric: 2 points of Armor Class
As mage: 1 point of Armor Class

Mystic Ambit Incant
Looks like a scroll with a blue rune.
As fighter: Temporary 1 point of Armor Class
As cleric: Large heal.
As mage: Large mana restore, first of blue mana and then if that's full it gives green mana.

Platinum Helm
As fighter: 3 points of Armor Class
As cleric: 1 point of Armor Class
As mage: 2 points of Armor Class

Torch
Will light up the level for 120 seconds.

Porkalator
Fires a spread of projectiles that turn enemies into small pigs if hit. The effect will eventually wear off. Does not work on bosses.

Flame Mask
A puzzle item that you find in the Guardian of Fire area and you place it in the right spot of Guardian of Ice to melt a big ice block so you can get the Fire Key. I'm pretty sure it'll do this automatically so you don't need to use this as an item, because I got the Fire Key without using this manually.

Wings of Wrath
Looks like a golden skull with golden wings. Lets you fly, and you can press the Home key (by default) to return to walking. Doesn't carry over from hubs though, so when you go to the next hub it's supposed to be gone. If you take a fatal fall but activate the wings you'll be saved! Make sure you have those flying keys bound to something you'll remember.

Any other items you pick up that aren't in this guide are puzzle items for the hub level that you're in, so whatever they are, just try to go find them all and you can get through the level. They're more walkthrough items so I'm not going to get to those because you don't need to know how to use them, you just need to find them most of the time.