T12 refers to the diameter of the tube. They are 1.5" diameter and i think are the standard in the USA.
In Europe T8s are more common (1" diameter). I believe they are also more efficient.
T5s are 5/8 of an inch and are the most efficient interms of output per watt. they are even better than compacts.
I think only metal halides have the edge over them.
