This article is from the Tolkien Newsgroups FAQ, by Steuard Jensen sbjensen@midway.uchicago.edu with numerous contributions by others.
[This supplements question V.F.1 of the Tolkien FAQ.]
The words "Orc" and "Goblin" are essentially identical in meaning.
In the introductory note to _The Hobbit_, Tolkien writes that
_Orc_ is not an English word. It occurs in one or two places but is
usually translated _goblin_ (or _hobgoblin_ for the larger kinds).
_Orc_ is the hobbits' form of the name given at that time to these
creatures.
Some have taken this and other comments in _The Hobbit_ (such as the
reference to "the big ones, the orcs of the mountains" near the end of
"Riddles in the Dark") to mean that "Goblins" were smaller and "Orcs"
larger. However, Tolkien did not generally make this distinction. One
clear example comes from the chapter "The Riders of Rohan" in LotR,
when the companions reach the edge of Fangorn:
Upon a stake in the middle was set a great goblin head; upon its
shattered helm the white badge could still be seen.
The white badge makes it all but certain that this was one of the large
Uruk-hai. In fact, it very likely that this was the head of Ugluk
himself, slain after a climactic fight sword to sword with Eomer at the
end of the battle. If Ugluk could be called a goblin, any Orc could.
 
Continue to: