File extensions in plain English?

Does anyone have or know of a good "plain English"-type explanation of file extensions (how they work, how you know which is which, what programs open which, etc.)? I get asked this question a lot, and I think it is complicated and confusing for many. I've looked around and found a few things but nothing great.

If nothing turns up, I may try to put together something myself, but would rather tap into something existing if possible. Thanks in advance!