They first appeared about the year 2475 of the Third Age, when they conquered Ithilien and destroyed the city of Osgiliath.
The Orcs and Uruks in the service of Barad-dûr, the folk of Mordor, used the symbol of the Red Eye of Sauron. The Red Eye was also painted on their shields. At least one, a guard, on the march with Merry and Pippin had a black knife with a long saw-edged blade, used by Pippin to cut through the ropes on his hands. These Uruks of Mordor referred to Sauron as the Great Eye, and Grishnakh was their Captain. They were all long-armed and crook-legged, not as tall as the Isengarder Uruks but larger than the Moria Orcs. They could see better in the dark than the Isengarders could.
The Uruks of Saruman used an S elf-rune wrought in white metal on the front of their iron helms. This could not stand for Sauron, as he did not permit use of his name by anyone, therefore it was clear this stood for Saruman. Their shields had a small white hand centered on a black field. Aragorn commented that their gear was not in the manner of other Orcs at all. Instead of curved scimitars, they used short, broad-bladed swords. Their great bows were made of yewn wood, in length and shape as those of Men. They also appeared different physically: greater stature, swart, slant-eyed, thick legs and large hands. Although they did not like the light of the sun, they could withstand it better than other orcs. Saruman promised them man-flesh as a treat. He aided them with his wizardry as well: when Aragorn, Gimli, and Legolas followed the party of Uruks who kidnapped Merry and Pippin, Saruman's will caused weariness of the heart for the pursuers and lent speed to the Orcs. Ugluk led the Uruk-hai of Isengard, and since they were the strongest he felt that he led the hobbit march as well, insisting on going back by way of Isengard. This was the group that slew Boromir.
The name Uruk-Hai has the element Uruk, which is a Black Speech word meaning Orc, related to the (Valinorean) Quenya word Urko (Noldorin Quenya: Orko) of the same meaning.
In the movies by Peter Jackson, Saruman appears to be the only one who created the Uruks. They are shown in the movie as being released from a kind of membrane in the mud deep under Isengard. These Uruks are sent after the Fellowship, and their leader is Lurtz, a movie-only character.
His Uruks include Pikesmen, bormal Uruk-Hai, Archers, and Berserkers. The Berserkers are commanders. They shave their heads and fill their helmets with human blood, so that when they put on the helmets the blood runs down their backs. They carry doubly-bent swords. Pikesmen, as the name suggests, carry pikes. Archers carry crossbows. Normal Uruks wield a basic sword. They also use bladed shields, as seen in Amon Hen during Aragorn's fight against Lurtz.
See also: Olog-hai