[The Half-Hobbit]

There is a very neat little site caled the Hypertextualized Tolkien, which has answers to questions like this...all backed up with proof. They say that Orcs and Goblins are the same thing. My own personal theory is that Orcs from Mordor are more likely to be called Orcs, while orcs from elsewhere are more likely to be called Goblins. however, I have no reason for thinking that...so I might be very very wrong.
anyhoo...here's the site http://www.daimi.aau.dk/~bouvin/tolkienfaq.ht