Traps usually consist of defensive structures such as Towers, but they can also be a particular spell such as Blizzard or a large mass of units A player lures enemy units into their trap with a retreating army The opponent's units then walk through the trap, usually dying before they have time to effectively respond
Nasty surprises left for unwitting adventurers wandering around the dungeon A trap might be a pit, a poisoned dart or even some evil magics triggered by walking on a specified spot Thankfully there is magic to locate these things beforehand, as well as the skills of searching and perception