Traps are a common danger in dungeon environments. From gouts of white-hot flame to hails of poisoned darts, traps can serve to protect valuable treasure or stop intruders from proceeding. Traps are (generally) intentionally constructed to either injure or delay those who encounter them.
The adventuring world is filled with dangers beyond dragons and ravening fiends. Hazards are location-based threats that have much in common with traps, but are usually intrinsic to their area rather than constructed. In other words, they are not usually intentionally created but are natural (or supernatural) aspects of an area.
Special terrains are great for making an encounter more interesting and dynamic.