It's a web-based e-mail service provided by Google. In order to limit numbers signing up for the service, you can only join if you're invited by an existing member.
GMail offers certain advantages over rival services (notably the 2Gb inbox) but is heavily criticised for the fact that all mail is electronically scanned. This is supposedly solely so that the advertising which funds the service can be appropriately targetted but many pro-privacy organisations have concerns about the potential for misuse of such a system:
http://www.privacyrights.org/ar/GmailLetter.ht m
People who don't need the massive inbox facility would probably be better advised to use a service such as Gawab, which offers a free webmail service which can also be used with an e-mail client (such as Outlook Express).
Chris