Long before Tolkein thought of constructing his own fantasy world of Middle Earth, he was a student of English literature, folklore, and mythology. Many of the creatures and demons who made their way into The Hobbit and The Rings Trilogy were direct descendants of creatures who had been scampering across the landscape of European imagination for centuries. One such species were the gruesome and violent Orcs. Whenever a granny was telling a tale to children around a fire she might need to pull out a generic monster or little non-human trickster.
These could range from comical and clever little leprechauns or dwarves all the way up to big scary monsters with little or no intelligence, like trolls or ogres. A goblin could be anything in between, and when Tolkein starting populating his fantastic world he conceived of a very specific type of goblin called an “Orc.” The similar term “Orcneas” had been used before in the Old English poem Beowulf. Orcs were a kind of anti-human. They had some intelligence, but it was all channeled into lines of thought which were completely hateful and destructive. For example, they hated everything and everyone, even themselves and whatever masters they served.
Although Orcs were intelligent enough to build things, they could only build destructive or ugly things, like terrible weapons. In general, Orcs were similar in size and construction to humans, so they were neither small like the clever hobbits, nor large like the giants of folklore. Despite being similar in size and shape, Orcs were corrupt, as if they were made up of spoiled blood and rotten parts. They usually had fangs, claws, and misshapen bodies.
The Orcs in the Ring Trilogy were sometimes creatures with a society and history and biology all of their own. Tolkein went through several ideas regarding the question of exactly what Orcs were and where they came from. These ideas ranged from being born of other Orcs; being the offspring of demons, sometimes with humans; being born from mud pits by way of dark magic; or being corrupted elves.