Enhancement bonus

An enhancement bonus represents an increase in the sturdiness and/or effectiveness of armor or natural armor, or the effectiveness of a weapon, or a general bonus to an ability score. Multiple enhancement bonuses on the same object (in the case of armor and weapons), creature (in the case of natural armor), or ability score do not stack. Only the highest enhancement bonus applies.

The most common type is the plus enhancement bonus (e.g.: +1 enhancement bonus), these can come in +1 to +10 varieties, the plus number indicates how much of a bonus it provides to each of the following:
 * Armor Class for an armor that has been magically enhanced
 * Attack and damage rolls for a weapon that has been magically enhanced
 * For shields, an enhancement bonus increases attack, damage, shield bonus to armor class and active damage reduction

In addition:
 * Weapons that have any + enhancement bonus will bypass DR/Magic.
 * Armor that has any + enhancement bonus is automatically considered Masterwork, and thus it isn't as cumbersome, reducing the Armor Check Penalty by 1.

Enhancements bonuses do not stack (for instance the bonus given by Dexterous Boots would not stack with the bonus given by the Cat's Grace spell).

Enhancement 'type' bonuses are also the most commonly used bonus type for some other things besides the standard plus bonuses such as:
 * A specific skill for some spells (like Jump) and some magic items (another more common kind of skill bonus is a competence bonus)
 * A specific ability for some spells (like Bull's strength) and some magic items
 * Attack speed (e.g. from Melee Alacrity items or Haste spell)
 * Run speed (e.g. from a striding item or a Haste spell)
 * Numerous others.