Normal Output Flourescent. This is the standard florescent tube. Tubes also come in Ho (high output) and VHO , very high output. Each requires their own ballast, based on their wattage, not tube length. ALso you might here about ODNO, over driven normal output. This is done by using a ballast to run 2,3 or 4 tubes of a specific wattage and wiring it in such a way as to powewr 1 tube, of hte same wattage, multipal times. This does not double the light output each time. Generally you can assume maybe a 30-40% increase in light output. This, ofcourse, makes the bulbs life shorter. Hope that helped