I was sent this question in email:

Why can't somebody just create a version of the software that gives you extra bitcoins?

... and couldn't find a good, non-geeky answer in the FAQs.  So I wrote one:

When you spend some bitcoins, the software on your machine has to prove to the software running on everybody else's machine that those bitcoins are valid.

How does it do that? Well, it is a little bit complicated; you've got to understand how bitcoins are created, and how they are traded.

First, how they're created: 50 bitcoins are created approximately every 10 minutes. Everybody who is trying to create bitcoins is in a race to try to find those 50 bitcoins; they are really hard to find, but, once found, it is easy to verify that, yes, indeed, your bitcoin software found them, so you get to spend them.