GMail has officially release their IMAP feature to every users. Check the screenshots for more information about IMAP feature on GMail. By default, they have activated IMAP, so in most cases, you won't have to change anything in the server side (in this case, GMail's mail server). The only configuration needed is in the client side, which is usually your favorite email client application. You can use GMail's Help to configure your application. For a definition and brief information about IMAP, you can check on Wikipedia.