Attack Actions

Certain maneuvers that combatants can perform are not standard actions, but are actual types of melee attacks. This means that if you get multiple attacks, you can make multiple attempts to grapple, disarm, trip, or sunder, or any combination of the above. You can also make these types of attacks when given an attack of opportunity.

This makes Improved Trip and Combat Reflexes an especially potent combination. Say an opponent tries to charge past you to attack your ally. As an attack of opportunity, you can trip your foe, preventing his charge, and allowing you an additional attack of opporunity on him (which you could then use to grapple, sunder, or disarm).

Source: 3.5 edition FAQ.