Microsoft has divided Windows Vista into a total of six product editions, all of which are tailored to a specific target group. Read more about Ultimate, Home Premium, Home Basic and Co.

You can find a basic introduction to all editions on http://www.microsoft.com/germany/windows/windowsvista/getready/editions/ultimate.mspx.
However, I also particularly recommend the Official Windows Vista Product Guidewhich offers a detailed comparison between the editions. Worth reading!
NOTE FOR NEW PUBLICATION: This article was produced by Sandro Villinger and comes from the Windows Tweaks archive, which has been built up since the late 1990s.