I think that most people believe that these terms refer to different things...
Different breeds or races possibly. It is simply not true. Tolkien himself has
said on several accounts that they are all the same thing.
From The Hobbit: The Hobbit or There and Back again
(Foreword)
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 to these creatures
As mentioned here, Orc is just a translation of Goblin. The Hobbit is after all
a book written for children and to have a book full of foreign terminology like
Orc would have confused them.
From The Letters of JRR Tolkien: Letter 131
Also the Orcs (goblins) and other monsters bred by the First Enemy are not
wholly destroyed.
Here it says Orcs and then it says Goblins in parenthesis.
The Letters of JRR Tolkien: Letter 144
Orcs (the word is as far as I am concerned actually derived from Old English
orc 'demon', but only because of its phonetic suitability) are no where clearly
stated to be of any particular origin. But since they are servants of the Dark
Power, and later of Sauron, neither of whom could, or would produce living
things, they must be 'corruptions'. They are not based on direct experience of
mine; but owe, I suppose, a good deal to the goblin tradition (goblin is used
as a translation in The Hobbit, where orc only occurs once, I think).
Again, it says that "Goblin" was used as a translation in The Hobbit for "Orc".
From The Hobbit: The Clouds Burst
The goblins gathered again in the valley. There a host of Wargs came ravening
and with them came the bodyguard of Bolg, goblins of huge size with scimitars
of steel.
Although it wasn't mentioned in this topic, this quote puts to rest that
Goblins are smaller Orcs.
From The Lord of the Rings: Appendix F
Orc is the form of the name that other races had for this foul people as it was
in the language of Rohan. In Sindarin it was orch. Related, no doubt, was the
word uruk of the Black Speech, though this was applied as a rule only to the
great soldier-orcs that at this time issued from Mordor and Isengard. The
lesser kinds were called, especially by the Uruk-hai, snaga 'slave'.
Uruk-hai are the same thing as Orcs. It was only applied to the larger
Soldier-Orcs, however. This does not mean that they were resistant to sunlight
or were some special breed. It merely means that by chance they were larger
than others of their kind. Saruman's Orcs were called Uruk-hai because they
were large not because they had were specially modified to resist sunlight or
were able to run longer distances.
Before I end this post completely against the notion that they are, in fact,
the same creatures, there is a reference in the Book of Lost Tales that
suggests a possibility that there was a difference between Goblins and Orcs.
This was of course written c. 1916-1917 so it could be irrelevant now but there
is always the possibility.
From HoME II: Tale of Tinúviel
...Nigh were the sad chambers where the thrall-Noldoli laboured bitterly under
the Orcs and goblins of the hills....
From HoME II: The Nauglafring
Moreover he gathered about him a great host of the Orcs, and wandering goblins,
promising them a good wage ...
Again those quotes don't say there was a difference but it is somewhat odd that
they are used as if they were different entities. Christopher Tolkien also
suggested a possibility that they were distinguished at times, in the
Index.
