The different dryer cycles as stated by Mary Marlowe Leverette on About.com, are air dry, delicate, permanent press, regular, and steam. All five cycles are utilized for different purposes and to dry different kinds of fabrics and materials in different ways. Dryers essentially inject hot air and withdraw moisture from clothing, but today there exists several settings for optimal drying.
The air dry cycle (or air fluff cycle) adds no additional heat and simply uses room-temperature air while tossing the contents of the drying machine around. Ideal for freshening dry-clean-only clothes or freshening up musty clothes that have been sitting in the cupboard for a while, this setting will come in handy sometimes but shouldn't be used to dry wet clothes as the temperature won't be high enough.
The delicate and permanent press cycles are intended for either delicate clothing, or for clothing that is naturally wrinkly and perhaps needs additional heat to minimize wrinkles. The regular cycle should be used on jeans, towels, sweatpants, or anything heavy duty that needs a lot of heat and a lot of drying, and finally, the steam cycle is ideal for clothes that don't need washing but need a little bit of odor and wrinkle removal.