A helpful guide to
Gmail can be found
here.
It's basically a new service from Google which has 1000Mb (1Gb) of storage space and no ads. The main difference between this service and others is that messages are not shown as you'd expect, they are grouped and you search for them as if they were web searches. Google believes you shouldn't have to delete old emails, just archive. That way, you never lose a piece of mail.
At the moment, it's being Beta'd, meaning it's being tested by the public. As they work things out, new features become available, but it's not yet ready for full use by the public. That means they can't let just
anyone sign up, so they let users to
invite other people. This lets Gmail to limit the numbers of users.
Currently they are being sold on
eBay for a high price by existing users, so Imogen's offer is a great one.
Edited to add: Prices seem to be $9.99 for one invite!