What do plane numbers mean?

I've tried googling this, but maybe the answer is so simple that no one ever thought that this question would be asked. I've been trying to learn a little bit about handplanes by reading online, and have yet to figure out ifthere is any logic to the numbering scheme of handplanes. My only guess is that it may be some type of measurement in inches, but that doesn't seem right since I think I've seen things in the 60's. Can anyone tell me what the difference between a Stanley #5 and a Stanley #7? Also, are the numbers consistent between brands? I really appreciate any advice.
Thanks!