POP vs IMAP email...

I currently use Roadrunner mail as my main address (@.cinci.rr.com), which I believe is POP. Right now I have it set up with Mail on both my iMac and Macbook. Whenever I receive a new message, the message will appear on both machines so I will see it on which ever computer I happen to be on. I'm just curious what IMAP email does differently. I just downloaded the new iPod touch software update so I will be setting up Mail on the iPod soon.