Demons are the most widespread race of fiends. The demons are chaotic evil by nature, and are native to the Abyss. Demons have no true rulers, though powerful demon lords are able to gain enough power and influence to gain control over sizable armies of demonic creatures.
D&D has long had unique demons of special power called demon lords (or Abyssal Lords in 2e). Each demon lord is a unique character, with its own history, powers, and abilities. Demon lords rule over their lesser, chaotic brethren largely by force of personality and threat of bodily harm, without any true hierarchy except that of who can kill whom.
In some products, it has been suggested that demons undergo a process of transformation from one type to another as they attain personal power.
Typed Demons and Named Demons
In OD&D and 1e, some demons were not given specific names, but rather referred to by "Type." Six types of "typed demons" existed ("Type I" through "Type VI"), though the succubus was named in OD&D, and Vrocks, Hezrou, Glabrezu, Nalfeshnee, Mariliths, and Balors were named in 1e.
Tanar'ri and Other Subtypes
In 2e, the words "Devil" and "Demon" were dropped by TSR from all rulebooks. Demons became "Tanar'ri," and all "Type" demons were given an official name based on their original suggested names.
In 3e, "Tanar'ri" was considered one subtype of demon. Other subtypes, such as Obyriths and Loumara, were created, enriching the history of the Abyss. In addition, some demons did not have a subtype -- these general demons were not members of any particular demon family.
Living Engines of Destruction
In 4e, demon subtypes were dropped, and all fiends underwent a new categorization. Demons became "living engines of destruction," without the need for any of the subterfuge or conniving of the devils, and inherited the more alien and bestial of the fiends, while some more subtle demons (such as the succubus) became devils instead.