There's no difference. Here is the explanation that Tolkien himself published after he had written The Lord of the Rings (meaning he added the following to a revised edition of The Hobbit).
'(2) 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, and it is not connected at all with our orc, ork, applied to sea-animals of dolphin-kind.' JRRT The Hobbit
Orc is Westron, 'goblin' is a modern word that (sometimes) translates it. This idea works for The Lord of the Rings as well.
The external history (looking at Tolkien's work through the years) is more complicated and confusing here, but this is the idea JRRT landed on for publication. Orcs come in different sizes, for example, but they are all Orcs. Or to translate that more completely into English: Goblins come in different sizes, for example, but they are all goblins.