The Uruk-hai are a race of monsters from Lord of the Rings that are supposedly a cross between Orc and Man made by Saruman in the name of creating a superior foot soldier for their armies. They were also meant to supplant the Orcs, and had nothing but contempt for their cousins, along with everything else.
The Uruk-hai were created by Saruman and there are suggestions that the Uruk-hai were the result of crossbreeding Orcs and Men.
The Uruk-hai were clearly meant to be the super-soldiers to the armies of evil, also likely meant to ultimately replace the inferior Orcs altogether. Indeed, Uruk-hai frequently displayed total contempt towards their lesser cousins, even killing and devouring them on at least one occasion. The Uruk-hai were bigger and stronger than Orcs, standing upright and also being less afraid of the sunlight, though it still affected their skin negatively.
The homicidal monsters were first encountered by the Fellowship at Amon Hen, where they ambushed them and managed to kidnap Merry and Pippin, as well as mortally wound Boromir. These Uruk-hai however, were later annihilated by Rohirrim led by Eomer. This would not be the end of Rohan's troubles with the Uruk-hai however, as Saruman soon unleashed all of his creations upon those lands. Forced to flee their homes, the people of Rohan sought refuge in Helm's Deep, but an undeterred Saruman simply sent the entirety of his forces to the citadel to lay waste to it and slaughter it's inhabitants.
Rohan's people fought valiantly, but the Uruk-hai's numbers were too great, and eventually they began to get overwhelmed. However, Gandalf and Eomer managed to arrive in time with reinforcements, unleashing a surprise attack that completely wrong-footed the Uruk-hai, also shattering their overblown confidence and causing them to panic. Fleeing in disarray and terror, they were massacred. Those that escaped the Rohirrim were killed by the Huorns of Fanghorn so as to ensure that they would not launch a second attack on Helm's Deep.
More Uruk-Hai would show up in the service of Sauron, aiding in the siege of Minas Tirith and also patrolling Cirith Ungol. In the case of the latter, the Uruk-hai ultimately came to blows with the Orcs also stationed there. The two sides annihilated each other.