CNET has a writeup for Microsoft here. Michael Horowitz has a blog entry for Computer World on "What you don't know about ..." the product here. It seems a bit redundant if you have a fully working third party program.
And then, there is something else, Windows Defender, an antispyware tool whose update seems to happen automatically almost daily and seems to take a while to install when noticeably packaged with other updates so that its install is watched. In Windows 8, it is upgraded to an antivirus program.
Apparently the enhanced Windows Defender for Windows 8 caused some "conflict of interest" with third party vendors however, making its use (or decision not to use), trickier, as explained in PC World y Brad Chacos here.
Indiana University has an explanation here.
In Windows 7 environments, I usually get pestered to run Windows Defender, which seems redundant.