Yep, it uses Unity for the client, and the server is a custom engine that uses Unity and Java. But before you get too excited about making your own, I should warn you that making an MMO in Unity is VERY hard, it stresses the capabilities pretty heavily, and it will take years of experience with the engine to get an MMO working well in it! Not to dissuade you, but just to warn you.
Unity's work flows were made for small games, and they don't scale to MMOs very well. However, their graphics engine itself works well for any sized game. So you just end up having to modify the stock work flows a lot. Which is easier said than done sometimes.
The upcoming Unity 5 has some big workflow improvements that might make MMOs more plausible for beginners soon, though!