Any encounter area (but frequently those within dungeons and ruins) can contain traps, fiendish devices both mechanical and magical in nature and designed to repel, harm and even kill the unwary. Many can be found and disabled before they are triggered, while some (especially magical traps) are nearly impossible to detect. Most traps allow some sort of saving throw that allows heroes to avoid or at least reduce the effect of the trap being sprung.
Normal characters have a chance equal to a roll of 1 on 1d6 (about 17%) to detect a trap if a search for one is made within the current encounter area. Thieves have a special ability to find and remove traps which supersedes this roll, based on their level (a first level thief can find/remove traps 20% of the time, while a tenth level thief can do so 63% of the time). Encounter areas can only be searched a single time, but all characters within the adventuring party will receive a chance to find any hidden traps (and if one is found, all thieves within the party will be granted a chance to automatically disarm the trap).