Basically Tolkien never could come up with a good way to have orcs with metaphysics he liked. He didn't believe a free willed being could be inherently evil, so at times he wrote that orcs had no free will and just did what Sauron wanted, essentially ants. Other times he thought that didn't fit and had them as people in a cruel situation who ended up cruel, but that was also unsatisfying for him. There isn't really one canon answer as to what an orc is.
