There’s a July 2015 Volume Licensing Guide released by Microsoft giving a broad introduction to both program licensing and product licensing.
It covers agreements for commercial, government, nonprofit, and academic customers as well as those aimed at software and service partners.
There’s a nice table on page 19 comparing the commercial programs which is a useful summary. The product licensing section of the document isn’t updated for Windows 10 yet, but again is a good overview of how the licensing works for each product.
Get this guide here: http://bit.ly/1MMvySK.