Well (Wierd... Nobody answered yet... ) it's all about the features.
Windows 7 Home Premium has features you would use at home (Media Center, etc..) while Windows 7 Professional has all the features of Win 7 Home Premium, as well as some you would need in a workplace (Virtual PC, Group policy editor, etc...).
The full list can be found here:
http://www.microsoft.com/windows/win.../versions.aspx
Um... Actually, that list isn't that full... I'll do some research. I've seen a better one before.
Edit: MEH! Clifford, you beat me