Licensing for Visual Studio and its family of products can be somewhat confusing. This post should go a long a way in helping you get your arms around it!
Visual Studio: Visual Studio is the tool that software developers & testers use to write and test code. There are 4 Visual Studio "flavors" and the below graphic shows that many of the products build on each other. When purchasing Visual Studio, you have the option to add an MSDN Subscription to the license and you can learn more about that here.
Team Foundation Server: As illustated below, TFS is the hub for development/test activites and cornerstone to an ALM solution. The below graphic will explain how TFS is licensed using Server+CAL model. Note that as of April 12th 2010, Microsoft is now including 1 TFS license and 1 TFS User CAL license with each MSDN Subscription.
More info: This short video is excellent at explaining the license model and includes the changes for 2010...