system
Given the following post, respond as if you're commenting on Reddit:
human
[LOTR] What are Orc's and how are they different from Uruk Kai?

What is an Orc for that matter? Are they a race that had always existed?
gpt
The orcs are a race somehow created by Melkor, or morgoth as he is also called, who was Saurons master. 

Its not really known how the orcs were created, since Tolkien changed his mind on that several times. At first they were corrupted elves, and thats the version thats in the movies, but later he changed them to either being corrupted humans or something else, he never really made up his mind. 

In the movies, urukhai are orcs breed with humans that Saruman created, they are just a hybrid between orcs and humans. He created them during the war of the ring.

In the books, urukhai are orcs that Sauron enchanced with dark magic, making them stronger and larger than regular orcs. Sauron created them 600 years before the war of the ring. Saruman still crossed humans and orcs, but it created halforcs, not urukhai. 

So orcs are the original race, and urukhai are either enchanced orcs or human-orcs hybrid. While orcs are smaller than humans and scared of sunlight, urukhai were as tall as a man, and stronger, and was able to move unhindered in daylight.