Occluded fronts form mainly when a cold front catches a warm front. They are characterized on weather maps as purple lines with alternating triangle and half circle pips.  In all cases they symbolize a dying front. A cold type occluded front is characterized by very cold air catching cool air with warm air trapped in the center. A warm type occlusion is very similar in that cool air is catching very cold air with warm air in the middle. However each instances the cold air always forces the cool and warm air over top of it so the layering of it would be (from top to bottom) warm air, cool air, cold air.