Help!

[franzferdinand4]

Someone please help me define the word temping. I shall give an example, of the word.

I got it out of the School of Rock :P

Jack Black: I got a job
Ned's g'f: doing what?
Jack: I do what Ned does, i'm temping.
Ned:Dewey, a subtitute teacher, is not a temp!

Help me define!