Orcs

Orcs are wicked creatures in service to the Dark Lord Sauron. To mount his conquest of Middle-earth, his agents gather massive armies to wage war against the Free Peoples.

The Mines of Moria
When the Fellowship of the Ring reaches Balin's Tomb they hear the blow of horns and several Orcs led by a Troll emerge and attempt to get inside. They manage to tear down the doors to the chamber and are slain by Aragorn and Boromir.

The orcs follow them to the Bridge of Khazad-dum where they break their lines to allow the Balrog a clear path towards the enemy. After the demon falls with the wizard Gandalf in the chasm bellow, the orcs continue their hunt for the Fellowship.

Amon Hen
The orcs manage to track the Fellowship to Amon Hen where Merry and Pippin bump into them. The creatures try to trap them before several are cut down by Boromir.

They leave the camp and a few other scouts shoot the warrior with hundreds of arrows. He briefly scares them away with his horn, but one of them finishes him off with another arrow. The two hobbits attempt to defend the wounded man but are quickly disposed of and taken as prisoners.

Road through Rohan
The horde leaves the ruins with their captives, believing them to have the One Ring, and heads to Isengard by taking the pass through the kingdom of Rohan. When they stop to rest, the hobbits ask for food and Uglúk reveals where they are going and tells them that they would have what they want there.

Several of the orcs are in the service of the wizard Saruman while the rest stand for the Dark Lord Sauron. Both bands hope that the prisoners have the One Ring in order to get it either to Isengard or to Mordor They are constantly on the run from the Rohirrim Riders led by Éomer, with whom they are later forced to fight with near the eaves of Fangorn Forest, where they are defeated and their fate is left unknown.

Preparations for War of the Ring
In Isengard, Saruman is amassing legions of orcs to destroy the kingdom of Rohan. These soldiers are sent by him to fight King Théoden's people at his fortress, being armed with huge pikes, battering rams, ladders and catapults.

When they arrive, they begin to chant in order to plant fear in the hearts of their enemies, blowing horns and carrying large pikes with skulls atop them. The orcs aided by Saruman's magic and their war machines break into the citadel and so starts the Battle for Helm's Deep. The Rohirrim flee from them and regroup. While Saruman's army attempts to break into the citadel of Hornburg, they are scared by the blow of a horn and the unexpected counter-attack by the king and his men. They are pushed back and soon Gandalf the White comes in to save his friends, clashing with the orcs, who either fall by his sword or retreat back to their master.

Trivia

 * In Ralph Bakshi's The Lord of the Rings, all Orcs come from Moria and have different variaties including gorilla orcs, mummy orcs, horned orcs, ent-like orcs and fanged orcs.
 * Their shields do not have the mark of either the White Hand (that stands fo Saruman) or the Eye (which stands for Sauron) so their allegiance is revealed only by their lines of dialogue.