The Uruk-hai made up a large part of Saruman's army, together with the Dunlendings and other human enemies of Rohan, and similar large Orcs also served as the elite troops of Mordor. They were faster than normal Orcs and could travel during the day without being weakened. They were not only faster but smarter, stronger and larger, though some were still shorter than Men. There are suggestions that the Uruk-hai were the result of crossbreeding Orcs and Men. Certainly, other creatures in Saruman's armies, and under his command in the Shire, appear to have been hybrids, though these "half-orcs" were as tall as Men and are never described simply as Orcs, as the Uruk-hai frequently are. Saruman's army of Uruk-hai fought against King Théoden of Rohan and his people at Helm's Deep.
The race of uruks, described as 'black orcs of great strength' first appeared about the year 2475 of the Third Age, when they conquered Ithilien and destroyed the city of Osgiliath. These were evidently of Sauron's breeding, but it is not clear that these uruks should be regarded as identical with the Uruk-hai, who could be a further 'improvement' to the race achieved by Saruman.
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 Uruk-hai of Saruman the White used an S elf-rune wrought in white metal on the front of their iron helms. It was clear this "S" stood for Saruman, because their shields had a small white hand (the symbol of Saruman) 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 yew wood, in length and shape as those of Men. They also appeared different physically: greater stature, swarthy, 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.
In Peter Jackson's The Lord of the Rings, 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 (special commentary on the DVD edition explained that they were trying to base the scene on a early description of Tolkien's that Orcs "worm their way out of the ground like maggots"). In the movies Uruk-hai are described as a crossbreed between Orcs and "goblin-men": this may be a dialogue error because in Tolkien's works "Orc" is a synonym for goblin. This description may however refer to humans corrupted by Goblins. These Uruks are sent after the Fellowship, and their leader is Lurtz, a movie-only character.
His Uruks include Pikesmen, normal Uruk-Hai, Archers, and Berserkers. The Berserkers are the shock troops. When they were first spawned a helmet filled with blood was placed on their heads, so that they were filled with a bloodlust for their enemies. They carry doubly-bent swords, and forgo any armor in lieu of agility, slaying foes left and right, completely devoid of fear and pain. Pikesmen, as the name suggests, carry long pikes, while archers carry crossbows. Normal Uruks wield a basic sword, cast from iron, simple but heavy, and deadly in an Uruk-hai's strong grip. They also use bladed shields, as seen in Amon Hen during Aragorn's fight against Lurtz.
The Uruks were also very efficient using siege equipment, and had been trained to use crossbows with deadly accuracy. The Uruks, like the lesser Orcs, seemed to not care of each other's presence, shown by the battering ram wielders at Helm's Deep, barging each other off of the thin bridge. The Uruks also hated the Orcs, believing they were a lesser being and often rioting, eg in the tower of Minas Morgul when Shagrat and Gorbag argued over Frodo's vest of Mithril and starting a mass war within the tower. The Uruks seem also to be able to control natural urges than the Orcs, eg the Orcs demanding to eat the hobbits they had captured, while the Uruks were protective. It would seem the only way Orcs were better than Uruks is in treachery, lying and being devious. The Uruks are also not seen to ever ride a mount, possibly due to size, weight and build (the Wargs which attacked the Rohan migration were ridden by trained Orcs).
Uruk-Hai is the original name of the one-man black metal band Burzum. There is also a Spanish black metal band called Uruk-Hai.