"The Tormented Stingers may resemble giant scorpions, but in truth they are moulded from the bodies of human sacrifices. The demons twist their victims into the stingers' distinctive forms by slicing open their chests and viciously mutilating their legs. Maddened by pain, these creatures can poison their prey with a single strike."— Deckard Cain(src)
Visually, they resemble large (human-sized) scorpions with their tails and pincers poisoned with lethal toxins. However, upon closer look, they turn out to be twisted, sadistically mutilated and literally bent beyond recognition human bodies, constantly experiencing agonizing pain that drives them to madness.
Despite their origin, they fall into the Beast monster type (Stygian Crawler subtype counts as Demon). Tormented Stingers usually attack in large numbers, often hiding in the demonic nests or underground. They hit relatively hard in close combat (with both Physical and Poison damage), but are very fragile and easily swept by any area damage attacks.
Large numbers of Tormented Stingers are a good source of combat healing with a high Life per Kill statistic. In return, non-elite Stingers award less gold, experience and loot than normal.