T5 just means the bulb is 5/12" in diameter. They are essentially high output fluorescent tubes, and, as far as I can tell, will probably become the standard in fluorescent fixtures. Depending on the reflector, they will generally give you more light per watt than PC (sorry, can't give exact numbers), but not enough to make the difference between a 96 watt and 36 watt fixture. Does that help?