What Is An Orc?

My youngest son is reading The Lord of the Rings for the first time, while I've been listening to an audio-book version during my work commutes. And our conversation this weekend drifted into discussing the books and then to the more specific question of what Tolkien meant by "orc" and whether it was synonymous with "goblin". I found an article, "Orcs Before Tolkien," that not only addresses possible origins of the word or term orc, but what I supposed is Tolkien's definitive statement of what he meant by the word:

In a note prefixed to later editions of The Hobbit, Tolkien clarified slightly (and confirmed that The Hobbit’s goblins and The Lord of the Rings’ Orcs were the same species):

“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.”