Unlike POP, IMAP offers two-way communication between Gmail and a user’s email clients. This means any changes made remotely instantly appear on the Web and elsewhere. While the email protocol has been around for years, it suffered from a couple of drawbacks which had limited its deployment: storage capacity and connectivity.
With IMAP all email is stored on the server, including sent mail, which can easily add up to several GB of data that ISPs (who used to run mail servers in the days before Hotmail) didn't want clogging up their servers. The other problem is that IMAP requires the user to remain connected, which back in the days of dial-up internet wasn't easy to arrange, but now is much less of a challenge.
Google will be rolling out the new IMAP features across the service over the next few days. In the meantime, the search giant has put together detailed instructions on configuring IMAP settings in both Gmail and a number of clients.