Amazon is currently offering 50% off pre-order for both the home and professional edition.

Here is the description from Wikipedia:
Windows 7 Home Premium
This edition contains features aimed at the home market segment, such as Windows Media Center, Windows Aero and touch-screen controls.

Windows 7 Professional
This edition is targeted toward enthusiasts and small business users. It includes all the features of Windows 7 Home Premium, and adds the ability to participate in a Windows Server domain. Additional features include operating as a Remote Desktop server, location aware printing, Encrypting File System, Presentation Mode and Windows XP

You  can also download a RC version of Windows 7 here.  This version will work until March of 2010.   Windows 7 is receiving a much better review than the previous Windows Vista.