A touch attack is a special attack that only needs to touch its target to have an effect. (A normal attack needs to hit its target solidly to have an effect. A character who swings a sword that glances off an opponent's shield has touched his target, but the swing is considered a miss since it could not inflict damage. In game terms, the attack roll was less than the target's AC.) Touch attacks can be either ranged or melee. One example of a ranged touch attack is an ettercap's web bolt attack. The web bolt merely has to touch the target to attach itself, binding the target to the ground.
A touch attack uses simplified attack bonus and armor class calculations. The attack bonus for a touch attack is generally base attack bonus + strength modifier. For touch attacks, the armor class of the target does not include armor, shield, and natural armor. All other AC modifiers, such as the size modifier, dexterity modifier, and deflection/dodge bonus (if any) apply normally.