What is this called? K+ FOR HELP

I think it's called a text mask.

Video:
http://tv.adobe.com/watch/short-and-suite/making-text-masks-in-after-effects/

alpha inverted track matte I'm pretty sure

this guy's right

really simple, you dont need after effects. a quick way is to make your text in ps

1 in a document the size of your video footage. black background with white text.

2 merge the layers

3 magic wand tool the white text

4 delete the white text you selected

5 you should be left with a black matte with a gap in the middle where the text was with transparent space where the letters were.

6 save as a .psd

7 import into fcp or the editing program of your choice

8 lay the footy down you want to appear in the text

9 lay the text matte .psd over top

bing bang boom render that n it should be good to go!

there are plenty of ways to do this, but thats always been a quick way that works for me
good luck

OP ought to take note of this.