Turbo question - what's the difference between a T3/T4?

Hey, what does the "T" rating on a turbo kit mean? Is it size? Power? A friend and I are talking about it. His mom has a 3000gt VR-4 (TT) and he was wondering what kind of upgrades he could do when/if he buys it from her.

So we started talking about turbo's, and I didn't know what the T4/T3/T2 stuff meant. I'd appreciate any info.

- Dzur