Taken from D20srd.org
When more than one character tries the same skill at the same time and for the same purpose, their efforts may overlap.
Often, several characters attempt some action and each succeeds or fails independently. The result of one character’s Climb check does not influence the results of other characters Climb check.
You can help another character achieve success on his or her skill check by making the same kind of skill check in a cooperative effort. If you roll a 10 or higher on your check, the character you are helping gets a +2 bonus to his or her check, as per the rule for favorable conditions. (You can’t take 10 on a skill check to aid another.) In many cases, a character’s help won’t be beneficial, or only a limited number of characters can help at once.
In cases where the skill restricts who can achieve certain results you can’t aid another to grant a bonus to a task that your character couldn’t achieve alone.
In melee combat, you can help a friend attack or defend by distracting or interfering with an opponent. If you’re in position to make a melee attack on an opponent that is engaging a friend in melee combat, you can attempt to aid your friend as a standard action. You make an attack roll against AC 10. If you succeed, your friend gains either a +2 bonus on his next attack roll against that opponent or a +2 bonus to AC against that opponent’s next attack (your choice), as long as that attack comes before the beginning of your next turn. Multiple characters can aid the same friend, and similar bonuses stack.
You can also use this standard action to help a friend in other ways, such as when he is affected by a spell, or to assist another character’s skill check.
This counts as your attack for the turn and you cannot make additional attacks unless your Base Attack allows it. If that is the case, this action takes your highest base attack bonus.