Grab your copy of this free guide from Directions on Microsoft. Yes, you need to register but it’s worth it. Here’s the blurb:

This 44-page report is a definitive guide to licensing SQL Server 2008 R2. Use it to get clarity about the type and quantity of SQL Server licenses your organisation needs and the best way to purchase them, for both conventional and virtualised servers. This guide provides detailed information about the eight editions of SQL Server and when they're most appropriate, product bundles that include SQL Server and when they make the most economic sense, the different licensing models you can choose between and how to decide, and much more.

Download the guide here.