Minecraft wiki entry for Trap_design, associated text: Trigger. The trigger is what starts the trap. It tends to be triggered by the target.
Complication. These make the trap more efficient, but are not strictly necessary.
Method. The method is the way the target is dispatched. The method is how the damage is dealt.
Bait. This is usually how you lure players in. This can be, for example, valuable objects, or it can be something highly visible, like black wool in the desert. This can be added to your trap or incorporate a small modification to a building being trapped.