This is my take on Tolkien's orcs, judging by what little detail is described in the books. Tolkien describes orcs as having many different breeds, bred for different purposes by their masters. He doesn't really differentiate between orcs and goblins like later fantasy fiction does, but "goblin" is mostly used about those in the north of Middle-Earth. These orcs are also a bit like mongrel dogs, as they've lived on their own literally for ages, which is why they're smaller than most of Sauron's orcs from Mordor.
The most prominent of all breeds are the Uruks of Mordor, as well as Saruman's own Uruk-Hai. Saruman's creatures are described as larger than most orcs and even taller than the older "model" of Uruk. I've tried to make the Mordor Uruks look more primitive and ugly than the Isengarders on purpose, to make both more distinct from each other. The same goes for their gear, which for the Uruk-Hais is described as being more like that of humans.
It is implied that there is a breed meant exclusively for labour, except when Sauron gets desperate enough to send them out to war as well. These "snaga" are the troops that Frodo and Sam travel with briefly.
There is also a small tracker-breed, called a "snuffler" in its sole appearance in Return of the King. They're described as having over-developed sensory organs, especially their noses.
The "common" orcs aren't described very well, but I reckon they'd look like a very mixed bunch, with a wide variety of skintones and features shared with the more distinct breeds.