G-mail is Google's take on web-based e-mail, like Hotmail or Yahoo Mail.
Webmail can be checked from any computer, so it's great for people on-the-go, and it's also great if your household has more computer users than available e-mail accounts.
Here's Google's
press release about it.
Google's entry into the market blows the crap out of almost every feature offered by any of their rivals (most of which have been constantly scaling back their services, for seveal years now, unless you join their deluxe "pay service", and G-mail looks to blow away even their "paid" versions too).
But, G-mail's still just in a "testing phase" right now, so only a very-limited number of people were allowed to sign up for it. So, it seems that some of those lucky people are willing to sell thier G-mail accounts on eBay.