IMAP vs. POP
Remember Guy Hacking Code In Hex Editor? He's got more differences between the two protocols. Frankly, I doubt you are interested in those. The difference that most people do care about is that the POP protocol works in a Connect -> Receive/Send -> Disconnect mode. This is fine for a lot of users. But, what if you could cause changes to happen on the fly? That's the IMAP email protocol.
With IMAP, you still choose how often your device connects to Google and downloads your mail. This is configurable for either protocol. The difference is, that as you do things to your email, like delete them, it automatically updates your server account as well. Why does it matter?
If you have your email set to download every 2 hours and it gets your new Gmail at 10:00am and then you read your email at 10:30am and delete a bunch of messages you don't need anymore, but don't hit "Send/Recieve" then your server email is not updated until 12:00pm when the 2 hour timer goes off. That means that if you sit back down at your computer and login to your Gmail account, all of those messages that you deleted are still there. With IMAP, every time you delete, it is deleted on the server. It is not like push email. Your messages will still only get downloaded based on a timer, not when you get email. But, what it does do is respond to your actions immediately.
Ironically, Windows Mobile 6.1 email sets up Yahoo email as IMAP automatically. If you want to do that from your regular PC, you have to pay extra. It's always free at Gmail, but you have to manually turn it on, and that is why Microsoft went with the basic version that will work for everyone instead.