Non-destructive text editor

I'm thinking of answering a job application thing, only they're saying that the applicants need to know how to work with non-destructive text editors; Word and Frontpage WON'T do.
What the hell's that?

Get to know vim.

It's a pain in the ass at first, but you'll love it later. It's one of those programs where you can figure out a little of it and get by and you learn new stuff all the time after that.