Your Listen check is either made against a DC that reflects how quiet the noise is that you might hear, or it is opposed by your target's Move Silently check.
In the case of people trying to be quiet, the DCs given on the table could be replaced by Move Silently checks, in which case the indicated DC would be their average check result.
Action: Varies. Every time you have a chance to hear something in a reactive manner (such as when someone makes a noise or you move into a new area), you can make a Listen check without using an action. Trying to hear something you failed to hear previously is a move action.
Try Again: Yes. You can try to hear something that you failed to hear previously with no penalty.
Special: When several characters are listening to the same thing, a single 1d20 roll can be used for all the individuals' Listen checks.
A fascinated creature takes a -4 penalty on Listen checks made as reactions.
If you have the Alertness feat, you get a +2 bonus on Listen checks.
A ranger gains a bonus on Listen checks when using this skill against a favored enemy.
An elf, gnome, or halfling has a +2 racial bonus on Listen checks.
A half-elf has a +1 racial bonus on Listen checks..
A sleeping character may make Listen checks at a -10 penalty. A successful check awakens the sleeper.
The character can pinpoint the location of an invisible creature, or detect an illusion with an auditory component.
Defeat Illusion: The character can automatically detect any illusion with an auditory component for what it truly is. No Will save is required, and the character doesn't have to interact with the illusion (but he or she must be able to hear its auditory component). Special: A character can use Listen to notice the presence of an invisible creature (generally opposed by a Move Silently check). If the character beats the DC by 20 or more, he or she can pinpoint the location of the invisible creature, though it still maintains total concealment from the character (50% miss chance).
Source: SRD 3.5 Combined Skills