RPGs

Tolkien

"But of those unhappy ones [elves] who were ensnared by Melkor little is known of a certainty. For who of the living has descended into the pits of Utumno, or has explored the darkness of the counsels of Melkor? Yet this is held true by the wise of Eressëa, that all those of the Quendi who came into the hands of Melkor, ere Utumno was broken, were put there in prison, and by slow arts of cruelty were corrupted and enslaved; and thus did Melkor breed the hideous race of the Orcs in envy and mockery of the Elves, of whom they were afterwards the bitterest foes. For the Orcs had life and multiplied after the manner of the Children of Ilúvatar; and naught that had life of its own, nor the semblance of life, could ever Melkor make since his rebellion in the Ainulindalë before the Beginning: so say the wise. And deep in their dark hearts the Orcs loathed the Master whom they served in fear, the maker only of their misery. This it may be was the vilest deed of Melkor, and the most hateful to Ilúvatar."
An alternative explanation, only hinted at in Tolkien's writings, is that orcs, like other evil creatures in Middle Earth, were created by dissonances caused by Melkor in the song of the Ainur that created the world. As the origin song is the source of much of what happens in Tolkien's creation, the two explanations may actually be the same.

Orcs fought as warriors in Morgoth's and later Sauron's wars. They are described as smaller and weaker than elves and men, though often more numerous. They hate dwarves, men and especially elves and fight them when they can, though they lose more often than they win; after all good triumphs evil in stories.

Later fantasy

True orcs

It is not crystal clear how orcs look like, but there is some common ground. Orcs have large lower canines, almost like the tusks of a boar, jutting out from the mouth. Tolkien's orcs have sallow skin, flat noses and slanted eyes; others are brown, gray, black or greenish and have hooked noses instead. All are ugly and filthy because beauty is not a quality orcs value; rather they hate beauty. They are scarred from their countless fights. They are bow-legged and have relatively long arms, sometimes reaching down almost to the ground.
Some orcs like Tolkien's cannot stand the sun. Half-orcs, usually half orc and half human, can, though they do not like bright light. Orcs prefer to live underground and come out only at night. They have keen night vision and an acute sense of smell, almost like the warg-wolves they like to team up with.
For food orcs prefer meat, not caring from which animal, humanoid or whatever creature. They obtain it by hunting or raiding. The idea of crafting, trading or toiling the soil as farmers is downright appalling to them.
They are a lazy lot, unwilling to learn or to work. When they have to, they do perform labor, often tunneling, mining and crude construction work. But their favorite pastimes are hunting and war. They don't care for the lives of others but will often spare captives so that they can employ them as slaves. Orc equipment, especially weapons, is made by their slaves or taken by force in raids. Their armor is a mishmash of bits and pieces, strapped together without finesse. When something is broken, orcs do not repair it, but replace it with some other loot.
Orcish science, art and culture are almost non-existent. Their craft is purely functional, their art made to frighten rather than to please; their knowledge aimed at success in strife. They do not have a language of their own but borrow from other languages. These they debase by sticking to simple grammar, a guttural pronunciation and a small vocabulary, except for insults and swearing.