In AD&D cosmology, Baatezu (which is both singular and plural, similar to "sheep") are the paramortals commonly referred to by mortals as "devils." In any case, they are the incarnations of the lawful evil alignment. Baatezu dwell mainly on the plane of Baator, which is also known as The Nine Hells.
The difference between a "Baatezu" and a "devil" is usually moot, but technically, any denizen of Baator may be rightly called a "devil," while Baatezu are those that fall within the races accepted as being part of the Baatezu hierarchy.
- Amnizu (being the bald 4' guards of the River Styx, who keep Baator safe from invasion)
- Cornugon (being the 9' whip-wielding elite forces of the Baatezu)
- Gelugon (being the insectoid masters of the frozen layer of Baator called Caina, second only to the Pit Fiends in power)
- Pit Fiend (being the 12' gargoyles, usually cloaked in flame, who serve only the mysterious Dark Eight)
Baatezu share a few abilities:
- Summon Baatezu (number and type vary)
- Immunity to fire and poisons
- Resistant to cold and acids
- Telepathy within 100 feet
And most have a handful of other abilities, besides.
In appearance, Baatezu are vaguely humanoid, usually in sort of a Gothic gargoyle kind of way. Baatezu females are infertile, although some males have been known to sire half-breed offspring with mortal women (which almost always results in the death of the mother). Baatezu normally reproduce by taking the souls of particularly evil mortals, which become the giant larva that infest the Lower Planes and mutating them into the lowest forms of Baatezu. A Baatezu that is killed anywhere outside of Baator will reform on Baator after a given amount of time, depending on the power of the Baatezu, unless they were slain by a holy weapon, or their corpse was eaten (not reccomended for mortals).
Baatezu are famous for their rigid social hierarchy, and, beyond the least forms of Baatezu, only a limited number of each stratum exists at any one time. Advancement usually comes about by proving one's own deviousness, especially at the expense of one's direct superior.
Baatezu have been embroiled in the Blood War with their chaotic evil counterparts, the Tanar'ri, or "demons" since they first met, shortly after the dawn of time.
Bonus politics of role-playing games
In first-edition AD&D, the word "Baatezu" did not exist, and the creatures were simply called "devils." However, thanks to the combined legal efforts of philistines, by the time second-edition came out, the more ambiguous term "Baatezu" was created. By 2000, when 3e was published, the D&D franchise had enough money, lawyers, and giant corporate owners that the game was allowed to exercise its 1st Amendment rights and put these beasties back in the "D" section of the Monsterous Manual, although "Baatezu" is still mentioned there, as their "proper name."
As an aside: in first-edition, the specific devil breeds had no names. They were simply "Type-I," "Type-II" and so forth. For second-edition, TSR pulled appropriate names from out of folklore and myth to attach to many of these supernatural critters, even as they were also assigning them a goofy group-name.