A couple weeks ago I worked in the Microsoft booth at the RSA Security Conference in San Francisco. I was there answering various and sundry questions about Forefront products, and in particular, questions about Forefront Protection for Exchange. Quite a few times I was asked, "So what are all the Forefront products?" On the following product pricing and licensing page they have a succinct list of the products and an explanation of the licensing for each. I've found it helpful.