The T basically denotes the model and size for Garret turbos:
T2
T25 - used on the Pontiac's and various Nissan's (Japan only)
T28 - common for twin turbo setups
T3 - common on Chrysler and Ford setups, good for about 300hp
T3 VNT - used on Chryslers, Variable nozzle technology (outdated turbo)
T04b - old diesel turbo
T04e - more efficient and newer than the "B"
TS04 - race turbo good for at least 600hp
T66 - common on Buicks that make 700hp or more
T88 - I saw this once on a RX7 that made 700whp
The T3/T4 is a hybrid, usually uses the T3 exhaust wheel and a T4 compressor wheel. Has better responce than the T4 but makes more power than the regular T3. Good for about 450hp. Low cost turbo because of older T3 turbine, but has best bang/buck.
Those are some common types, starting with the smallest at the top.
