the 'T' number for fluorescent bulbs just refers to the diameter of the tube in eighths of an inch, so a T8 is 8/8 or one inch diameter, the pin arrangement at the ends of the tube are same (medium bipin) for T8, T10, and T12, but different for T5, which is miniature bipin
T5s and T8s are not interchangeable, so you will need to get the one that matches your fixture, and if T5, either High Output or not
watts are standard, e.g. a tube of a specific length will be of a certain wattage (but different for T5, T8, T5HO)
for appearance, look for Kelvin (or just K) rating and 'full spectrum', a full spectrum bulb of around 6500K approximates daylight, at least as best as can be achieved with fluorescent lighting, which has interupted spectral output rather than true full spectrum like incandescent sources, higher K ratings will be 'cooler' in color (bluer) and lower K ratings will be warmer (yellow, orange, red)
Last edited by Quantum; 03-18-2012 at 03:25 PM..