Twin Tube Lamps:
A CFL that has two parallel tubes are called Twin Tube Lamps. They are designed to fit into lamps, task lights, recessed ceiling lights and wall lights. This type of lamp bulb is also called a "Biax" or "Dulux" lamp.
Quad Tube Lamps:
Quad Tube Lamps give nearly the same light output as twin-tube CFLs of equivalent wattage. They are only half the length, however. This type of bulb fits better in smaller lamps and similar applications. They are also known as "Double Biax", "PL Clusters" or "PLC" lamps.
Triple Biax Lamps:
These CFLs generate even more light in a shorter bulb. Because they provide a high amount light output from a bulb that takes up a very small amount of space they are commonly used in fixtures designed for incandescent bulbs, such as table lamps.
The shape of a spiral lamp is a continuous tube in a spiral shape. The similarity in shape and light casting qualities to a standard incandescent bulb make this the most popular model of compact fluorescent lightbulb.
F lamps have two twin tubes, aligned top to bottom instead of side to side like a Quad tube making the shape of the bulb resemble an F. This type of CFL is commonly used for task lights and low profile recessed fixtures.
Circular (Circline) Lamps:
Circular lamps were designed for reading lamps and other lamps which use a circular light. There are some individual brands that give the "cool white" light found in a typical fluorescent. Other circlines available, however, have electronic ballasts and give warm light so it is important to make sure you buy the right CFL for what you are lighting.