What’s the difference between the editions?

Windows 7 will be available in six different editions, but only Home Premium, Professional and Ultimate will be widely available at retail. The other editions are focused at other markets, such as the developing world or enterprise use.[1] Each edition of Windows 7 will include all of the capabilities and features of the edition below it.

A great Comparison chart by Wikipedia:
http://en.wikipedia.org/wiki/Windows_7_editions#Comparison_chart