In the past (and maybe still) some of Microsoft's academic compilers were only for non-commercial use. Some would generate an exe file that wouldn't even run unless Visual C++ was installed on the target machine!
What can I do with the Express products?
Learn how to program using a streamlined, lightweight development environment with built-in tutorial content
Evaluate the .NET Framework for Windows and Web development
Create fun and interesting applications for their personal enjoyment or to share with their friends, or even commercially distribute your programs.