Dodge bonus (not to be confused with Unarmored bonus) is a bonus to Armor Class, and sometimes Reflex saves, resulting from physical skill at avoiding blows and other ill effects.
Dodge bonuses stack with all other bonuses to AC, even other dodge bonuses.
Dodge penalties also stack, and a character's dodge AC can be negative.
Any situation or effect (except wearing armor) that negates a character's Dexterity bonus also negates any dodge bonuses or penalties the character may have (for instance, you lose any dodge modification to AC when you're flat-footed).
Dodge bonuses apply against touch attacks.