We're not going to get into the technical side of it because, quite frankly, we don't understand it all. The Unofficial Apple Weblog (TUAW) has an extensive hands-on and gallery walkthrough (see below) of the $40 program, and found the software satisfying overall, especially for use with Steam. (You can view a compatibility list here.) There's also a 30-day free trial available.
Gallery: TUAW's Hands on with CrossOver Games