What is it called? How do I do it?

I'm not sure what it is called or even if fcpx can do it, (maybe a different kind of program) but hopefully you guys can point me in the right direction.
So lets say I have guy talking and moving around, he has a plain white shirt on. How can I put a video on top of the white shirt and it won't bleed outside the shirt or onto his skin etc... and will follow and stay within the confines as the guy walks around? I'm certain I've seen this effect before…

Thanks!

SliceX powered by Mocha.
Lots of fun!

www.coremelt.com/products/slicex-powered-by-mocha.html

I would also suggest you mark some black x's on the shirt, so you have really clear tracking points in post.

Another way to do it would be to have him wear a chroma key green (or blue) shirt and just key it out.

Which method depends on the environment, and how you want to put video on the shirt area.

This is just basic compositing.

I'd agree entirely with using a prominently-coloured shirt and keying that colour (even keying it back to white, if that's what you want). I recently colour-replaced a shirt in a shot: the original was a scrappy off-white, but distinct enough from the talent's skin tone and background that I could pull a reasonable key off it. Lighting limitations were the biggest problem, and the resulting key isn't great. However, it works well enough in the context of a training film for teachers: (starts ~25 seconds in).

As ever with keying, it really helps if you can test your footage for lighting issues as you're shooting - a laptop on set will save you hours of rotoscoping in post!

Yes, if you are shooting green screen (anything chorma key), always have a Mac laptop and VeeScope Live running on it.