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. A 20-level expansion for Doom II created by both id designers and hand-picked community members was released in the form of the Master Levels for Doom II.
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 with the franchise, as used in the name of the Classic Doom for Doom 3 mod.
id Software continues to distribute, exploit, and in some cases develop the classic line of Doom products. The Xbox 360's "Xbox Live Arcade" platform played host to ports of Doom and Doom II, the latter including a new 9-level expansion episode entitled No Rest for the Living. Doom 3: BFG Edition and Doom Classic Complete both include the classic series, in the former case together with the newer Doom 3 games. These are further available as backward compatibility programs on the Xbox One, and are included with some editions of Doom (2016) on that platform. Doom Classic experienced additional development in 2019 and 2020.