Almost every gmail post I've seen has someone posting to ask what gmail is- here's a link to google's info page. Please read that for your answer. And the reason you need an invite is because it's still in the "test" stage so it's not available to everyone yet. Hope this helps!
P.S. Sorry this isn't a deal or a freebie!
http://gmail.google.com/gmail/help/about.html#signup
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