In "The Lord of the Rings" series, orcs are servants of Sauron, the enemy of all free people. Orcs are malicious and subservient and serve as an example of the corruption and decay Sauron desires to bring upon the world. More practically, orcs make up the rank and file of Sauron's army.
The first orcs were elves tortured and broken by Morgoth, Sauron's master. By the time "The Lord of the Rings" takes place, orcs are so far degraded that it is difficult to see they were ever elves. The way that evil twists good things like elves into hollow mockeries like orcs is a major theme of "The Lord of the Rings." Sauron's orcs live in the barren and poisonous land of Mordor, and one point raised in "The Lord of the Rings" is that all life in Middle Earth would exist in the same unwholesome way if Sauron were victorious.
Orcs are not very intelligent; they gravitate naturally towards evil lords, so they make great minions. In "The Lord of the Rings," Sauron gathers huge numbers of orcs to be his foot-soldiers and Saruman breeds orcs into Uruk-hai to create his army. There is a large number of evil men that fight for Sauron and Saruman, but orcs make up the bulk of each army.