Uruks or collectively Uruk-hai (a Black Speech term meaning "Orc-folk") are a race of Orcs that are taller, stronger, and more powerful than their ordinary brethren. They are essentially the "super-soldiers" of Sauron's army. Unlike most Orcs, Uruks have no fear of the light of the sun. It has been suggested that they have the blood of Men in them or that they were Orcs crossbred with Half-orcs and Goblin-men to perfect the mix of racial traits. Whatever their beginning, they are among the most formidable warriors in the legions of the Dark Lord and the White Wizard Saruman.
Uruks come in two main varieties: The Black Uruks of Mordor were the creation of Sauron. They made up a significant portion of his armies and aided in the conquest of Ithilien and the sack of the city of Osgiliath. The later "fighting Uruk-hai" of the War of the Ring were the creation of Saruman, through dark alchemy and breeding experiments intended to improve upon the race of Uruks. They dwelt mostly near Isengard and were the Wizard's tools in the attempted destruction of Rohan.
A list of all known Uruks: