It introduced the concept of playing over a network together ("cooperation mode") or against each other ("deathmatch mode") into the genre of first person shooter games.
The source code of DOOM was published by ID Software in 1997 under a proprietary license. The game was then ported to various other operating systems. In late 1999, the source code was re-released under the terms of the GPL.
A sequel, DOOM II, was released at the end of September 1994. Doom II consisted of thirty regular levels, plus two "secret" levels, which nearly duplicated episode 1 level 1 and episode 1 level 9 of Wolfenstein 3D. The engine and gameplay were the same as Doom, however there were some different weapons and monsters.
There are several websites still in operation with information and news about DOOM and DOOM II. Some of the larger sites are http://www.doomworld.com/ , http://www.doomcenter.com/ , and http://www.newdoom.com/ .