From their site FAQ section
What’s the difference between the Microsoft software on DreamSpark and the Microsoft products I can buy in the store?
Nothing! DreamSpark offers the same completely loaded Microsoft software that professional developers pay good cash to purchase. However, through DreamSpark, we’re providing this software directly to students like you at no charge, to advance your learning and skills through technical design, technology, math, science and engineering activities. We want to give you the chance to explore the software tools used in business today to help prepare you to be a member of the next generation of developers.
Through dreamspark you can get
- Windows Vista Business
- Windows 7 Professional
- Windows 8 Pro
- Windows 8.1 Pro
And they are the same as commercial versions of Windows. But if you have one key for one Windows, you can not enter the same key for another one! Also you cannot enter a key for Windows 7 Proffesional to a Windows 7 Ultimate. (you must have key for that version of Windwos)