The original Doom games by id Software are Doom (with its shareware version, Doom: Knee-Deep in the Dead) and its continuation Doom II: Hell on Earth. Additionally, Doom itself was expanded by part of the id Software team and released as The Ultimate Doom, and a couple of megawads by fan community members were published in full IWAD format by id Software as Final Doom, as a semi-official continuation of the games.
The Doom games were originally developed on NeXT computers and released for MS-DOS, and then gradually ported to various other systems. The original Doom games are easily distinguished from id Software's newer series of games starting from Doom 3. The name classic Doom can be applied to make this distinction from the newer games associated to the franchise, as used in the name of the Classic Doom for Doom 3 mod.