College students. But in practice, anyone can get it, so far as you know someone who had a .edu email address.
Microsoft has deliberately made it easy for people to buy Win 7 cheap so they can sell more copies. Kinda like how they had these low priced “student editions” of Microsoft Office, which practically anyone could buy.
Someone started a thread here about getting a .edu email address, if you don't have a child in university.
http://www.freerepublic.com/focus/f-chat/2369537/posts