The main difference is that, with IMAP, everything stays on the server while POP always downloads and operates on local files, even if sometimes it pretends otherwise. If you only ever use Gmail on one machine you perhaps won't see any big difference, but if you were using several different machines to access your Gmail via POP (eg. home and work) then each one would individually download copies of each message when you logged on and everything you did in your mail client would apply only to the mail you've downloaded. So if you read all your email at work, then go go home and fire up the email client, all the email you read on the work machine is downloaded again and is marked as unread, because your home machine doesn't know any better - it's not in contact with the machine at work only the server. With IMAP, when you read the email at work the message is marked as read on the server. When you come home and fire up your mail client it can see that the mail is marked as read. Other advantages for IMAP is that apparently the tagging/labels stuff will work in mail clients.