Essentially what it is - is a partial replacement to Windows Live OneCare - Microsoft's current paid subscription anti-virus, anti-malware, firewall, maintainance and backup tool. This product will no longer be sold after June 30 and will be instead replaced with other different components. Microsoft will use Windows built in Firewall, Maintainance and Backup with Windows 7 by default and the new product called Microsoft Security Essentials will only be an anti-virus and anti-malware tool. It will not be included with Windows 7 - you have to download it seperately (though I'm assuming Microsoft will make it very easy to get).
The Tool disables Windows Defender and takes over it's role as Malware protector. It's very fast to load, very quiet and very lightweight. I've installed it on my system - here is a screenshot of the program.
With a program like this installed on your system - there really is no reason to buy Norton or McAfee anti-virus products - they are expensive and just bloat your system. If you practice safe computing and have your Windows Firewall turned on, backup your files and keep UAC turned on - then this is all you really need for anti-virus in Windows 7. It will also work on Windows XP and Vista too :)
