The Pit Lords are a class or rank of a powerful race of demons called the annihilan, recruited by Archimonde the Defiler to serve the Burning Legion. Their leader, the most feared of them all, was Mannoroth the Destructor.
The Annihilan Pit Lords are described as engines of destruction. They are driven by pure rage and bloodlust. Their bodies resemble massive lizards, with four powerful limbs, a tail, and wings sprouting from their backs (the wings are vestigial and occasionally used for defense - even with eredar magic, Pit Lords are far too heavy to fly). Their upper torsos are more humanoid, with a large chest, two arms and a large horned head. Their mouths are filled with rows of teeth, their eyes are on fire and even their tongues are studded with fangs. The preferred weapon of the Pit Lords is a massive double-bladed sword. Though they may look fat and lazy, they are unimaginably powerful.
The Pit Lords who serve under Mannoroth are some of the most cruel, barbarous butchers to ever roam the trackless wastes of the Twisting Nether. These hulking engines of hate and death love only to kill and bring sorrow to all living creatures. Fanatically loyal to Mannoroth and the Burning Legion, the Pit Lords will stop at nothing to see the will of the Burning Legion upheld.
At some unspecified point in time, Archimonde recruited the Annihilan into the Burning Legion, an invitation the Pit Lords readily accepted. This would have occurred somewhere between twenty-five and eleven thousand years ago; the Pit Lords were a major integrated part of the Legion by the War of the Ancients ten thousand years ago.
Mannoroth's blood was used to forge a blood-pact with the corrupted orcs on Draenor, making their corruption permanent - only one clan, the Frostwolves, escaped. During the Burning Legion's second invasion, Thrall and Grom Hellscream defeated Mannoroth, and broke the blood pact he had forged and thus freed the Orcs from demonic corruption. Hellscream did not survive the encounter.
After Mannoroth's death, Azgalor became the new Demon King of the Pit Lords.
Though the blood of both Mannoroth and Magtheridon has been used to corrupt the orcs, it's not clear whether this is an ability inherent to all Annihilan, or simply to certain powerful individuals. Since there are few Pit Lords willing to submit to experimentation, it's unlikely the question will be answered any time soon.
Nothing is known of the culture of the annihilan, but it likely is one where raw power is valued above all else. While Mannoroth is certainly quite powerful, he probably rose to become their leader through his cunning and magical knowledge - he is said to have been as skilled as a mid-level eredar. No other Pit Lord has shown particular affinity for the arts arcane.