I've often wondered the same thing, and I'm sure Alcuin would be able to elucidate the distinctions far more competently than myself (based on some of the impressive discoveries he made in the
Gandalf's Mark thread). However,
Tolkien does have a little to say on the matter...
The Letters of JRR Tolkien - #25: To the editor of the 'Observer'
And why dwarves? Grammar prescribes dwarfs; philology suggests that dwarrows would be the historical form. The real answer is that I knew no better. But dwarves goes well with elves; and, in any case, elf, gnome, goblin, dwarf are only approximate translations of the Old Elvish names for beings of not quite the same kinds and functions.
I'm unsure what
Old Elvish in this particular context means. If it is extrinsic to
The Legendarium than Alcuin's going to have to be your man as you'll have to seek elsewhere in terms of
Old Elvish Translations. However, if
Old Elvish is simply a reference to the Quendi (and the Avari) and thus
Quenya translations; then
goblins is just a Quendi translation for
Orcs and there is no difference - which is a lazy albeit clever way of Tolkien avoiding philological and grammatical corrigendums in his works. He does mention that the
Istari's and the Dwarf names are from the
Elder Edda which contained pagan poems of Scandinavian origin and constituted a majority of the Norse mythology (just as a side-note

).
This likely would be the most compelling evidence that Tolkien considered
Orcs and
Goblins to be one and the same:
The Letters of JRR Tolkien - #131: To Milton Waldman
Also the Orcs (goblins) and other monsters bred by the First Enemy are not wholly destroyed.
Tolkien literally equates the two in the extract above.
However, the quote below explains all:
The Letters of JRR Tolkien - #144: To Naomi Mitchison
Orcs (the word is as far as I'm concerned actually derived from Old English orc 'demon', but only because of its phonetic suitability) are nowhere clearly stated to be of any particular origin. ... 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)...
Tolkien believed that
orc may have been an Old English translation for
demon and contended that the usage of
goblin was a translation of this word in
The Hobbit; which conincides eloquently with the translation being Elvish. So I
assume when Tolkien stipulates that it is an Elvish translation he intends it to be a Quenya or
Sindarin translation (it apparently is
orch as singular and
yrch as plural in Sindarin). In Quenya it is translated as
urco (stem
urcu-) and has the same translation for
goblin. The problem however, is that
rc does not exist in Quenya and thus it is correctly
urko (stem
urku-). Tolkien comments about the origin of the word
orc in
The War of the Jewels:
The History of Middle-earth XI: The War of the Jewels - Quendi and Eldar: Appendix C. Elvish names for the Orcs
The word used in the translation of Q [Quenya] urko, S [Sindarin] orch, is Orc. But that is because of the similarity of the ancient English word orc, 'evil spirit or bogey', to the Elvish words. There is possibly no connexion between them. The English word is now generally supposed to be derived from the Latin Orcus.
So
orch in Sindarin is translated as
orc due to its similarity from the Old English
orc which, apparently, derives from the Latin word
Orcus which means
death,
the Lower World or
whales. Someone may need to clarify this one...
Tolkien then further delineates one difference between the two:
The Letters of JRR Tolkien - #151: From a letter to Hugh Brogan
Your preference of goblins to orcs involves a large question and a matter of taste, and perhaps historical pedantry on my part. Personally I prefer Orcs (since these creatures are not 'goblins', not even the goblins of George MacDonald, which they do to some extent resemble).
Essentially Tolkien used the words interchangeably as they meant the exact same thing.
Orcs are g
oblins and vice versa. It appears that he changed his mind in the period between the creation of
The Hobbit and
The Lord of the Rings and decided his personal preference for the term
Orcs;and then later equated the two in Elvish (and other) translations.
Cheers,
Úlairi.