Tainted orcs are very similar to the green orcs of the Horde. They are orcs corrupted by demonic energies.
Tainted orcs once cultivated society on the dying world of Draenor. Then the Burning Legion's dark magic hit them and corrupted the target. Attracted to the world of Azeroth through a dimensional gateway, they were manipulated into fighting against human nations. Recently, most orcs threw off the shackles of demonic influence and now strive to recover their lost heritage. Some orcs remain under the thrall of the Burning Legion, however, to the point where they are twisted into mockeries of their former selves.
After their defeat in the Second War, tainted orcs suffered from lethargy produced from the lack of demonic energy. Thrall, an uncorrupted orc, was able to rekindle their original shamanistic ways, and many of them threw off the shackles of demonic influence and now strive to recover their lost heritage. During the Third War, Grom Hellscream slayed Mannoroth, thus breaking the bond that kept the tainted orcs corrupted. From then on, tainted orcs were able to return to their original ways. However, some orcs remain under the thrall of the Burning Legion by their own decision, to the point where they are twisted. Fel orcs are the ultimate step in being corrupted by demonic energies.
Tainted orcs appear barbaric and brutal to other races. Nevertheless, they are also noble and proud with their lifestyles. In war or battle, a tainted orc can channel his inner fury and become truly fearsome in battle.