Your senses allow you to notice fine details and alert you to danger. Perception covers all five senses, including sight, hearing, touch, taste, and smell.
Perception has a number of uses, the most common of which is an opposed check versus an opponent's Stealth check to notice the opponent and avoid being surprised. If you are successful, you notice the opponent and can react accordingly. If you fail, your opponent can take a variety of actions, including sneaking past you and attacking you.
Perception is also used to notice fine details in the environment. The DC to notice such details varies depending upon distance, the environment, and how noticeable the detail is. The following table gives a number of guidelines.
Most Perception checks are reactive, made in response to observable stimulus. Intentionally searching for stimulus is a move action.
Yes. You can try to sense something you missed the first time, so long as the stimulus is still present.
Race Elves, half-elves, gnomes, and halflings receive a +2 racial bonus on Perception checks. Creatures with the scent special quality have a +8 bonus on Perception checks made to detect a scent. Creatures with the tremorsense special quality have a +8 bonus on Perception checks against creatures touching the ground and automatically make any such checks within their range.