It seems like Microsoft has introduced quite a bit of confusion with all the different Vista SKUs that have been released.  XP really only had two major version: Home and Pro.  However, Microsoft began to even skew the lines with XP because really there were other smaller versions of it as well: XP Media Center, XP Tablet PC, XP x64, XP x64 Pro, and XP Embedded.  But it wasn't an awkward decision when a customer was picking which version they would purchase … typical home users bought "Home Edition" and businesses and power-users bought "Professional Edition."  A lot of gray area has been introduced with the six versions of Vista:

• Vista Starter – Limited to emerging markets
• Vista Home Basic – Intended for budget users with low needs
• Vista Home Premium – Covers majority of home consumer market
• Vista Business – Designed for small businesses
• Vista Enterprise – “Premium” business edition only available to customers participating in Microsoft's Software Assurance program
• Vista Ultimate – Contains the complete feature-set and is aimed at enthusiasts

All editions except Windows Vista Starter support both 32-bit (x86) and 64-bit (x64) processor architectures.

Edition Comparisons:
If you want a 30,000 foot view of what the differences are in "marketing jargon" … Microsoft has posted this comparison: 
http://www.microsoft.com/windows/products/windowsvista/editions/choose.mspx.

However, if you want a more in-depth look at the actually features that are included or the limitations of the various versions … here is a more complete comparison from the Windows Vista Product Guide: