Vampirism turns you into a vampire. It gives you extra power.

The only way to become a vampire is to have Sanguinare Vampiris for three days. Sanguinare Vampiris can be cured before spending three days with it, but Vampirism is notoriously hard to cure.

Your eye color is changed to red.You can feed on sleeping bodies.You might have the power to summon a death hound. (Requires Dawnguard)You gain extra skills.

You will take sun damage (Except for Stage 1 Vampirism), but you must stay inside. Otherwise, you will see the message "Your vampire blood boils in the sun."Three members of the dawnguard can encounter you. (Requires Dawnguard)While in vampire lord form outside in the day, your attributes are decreased by 60.

If you side with Volikhar, you gain the power Vampire Lord. To leave that form, cast Revert Form. While in Vampire Lord form, your magicka is greatly better. (Females of that form are called Vampire Ladies) They can summon a gargoyle to fight on their side.

Dawnguard VS Player Vampire: Three members of the Dawnguard attack you. (Requires Dawnguard)

NON-BUGS:
Unlike Bloodmoon, you can't be a vampire and a werewolf at the same time.
CURRENT ISSUES:
At stage 3 vampirism, the effect has a 75% magnitude instead of a 100% magnitude.
Ebony and Blades Shields are invisible.
Nords and Khajiits don't get the glowing yellow eyes after Dawnguard is installed.

If Hearthfire is installed, you can place a sleeping coffin in your manor's cellar.
You will be considered undead if you have vampirism.