Source: Player's Handbook
You can respond quickly and repeatedly to opponents who let their defenses down.
Benefit: When foes leave themselves open, you may make a number of additional attacks of opportunity equal to your Dexterity modifier. For example, a character with a Dexterity of 15 can make a total of three attacks of opportunity in a round - the one attack of opportunity any character is entitled to, plus two more attacks because of his +2 Dexterity bonus. If four goblins move through the character's threatened area, he can make attacks of opportunity against three of the four. You still only make one attack of opportunity per enemy. You may also make attacks of opportunity while flat-footed.
Special: The Combat Reflexes feat does not allow a rogue to use her opportunist ability) more than once per round.
Normal: A character without this feat can make only one attack of opportunity per round and cannot make attacks of opportunity while flat-footed.