Steamworks will provide auto-updating, achievements and support for the Steamworks Cloud, which helpfully stores your data online. On top of all that, there's the tantalising promise of "and more." Both retail and digital editions of Cod Blops use Steamworks, meaning you that no matter where you buy your copy, you'll be able/required to add it to your Steam account and play through the Steam client.
Call of Duty: Black Ops is due out for PC, Xbox 360, PlayStation 3, Wii and DS on November 9. To see if your PC can handle it, check out the system requirements.