Short answers, pulled from the story.
Doom was released on the 10th of December 1993, developed and published by id Software for MS-DOS. The core development team consisted of five people: programmers John Carmack and John Romero, artists Adrian Carmack and Kevin Cloud, and designer Tom Hall.
Doom sold an estimated 3.5 million copies by the end of 1999 across all versions. In its first year, estimates ranged from 140,000 to around 200,000 copies of the full game, while the shareware version was distributed more than one million times by May 1994.
The Doom Bible was the design document written by id designer Tom Hall in November 1992, detailing a science fiction horror plot in which scientists open a portal to a demonic alien invasion. John Carmack rejected its emphasis on story, believing gameplay and technical innovation mattered more. The rest of the team sided with Carmack, and Tom Hall was fired in July 1993.
Eric Harris and Dylan Klebold, who carried out the Columbine High School massacre on the 20th of April 1999, were both avid Doom players. Harris wrote in his journal that the killing would be "like playing Doom." A widely repeated rumor claimed he had built a custom level based on the school to practice for the attack, but investigators found no evidence this was true.
Doom used a technique now called 2.5D graphics: the environment appears in a 3D perspective, but movement is restricted to a two-dimensional plane, with enemies and objects rendered as 2D sprites. Unlike Wolfenstein 3D, which limited walls to right angles, the Doom engine allowed walls and floors at any angle or height, and used binary space partitioning to render only the visible portion of a level at any moment.
Doom is credited with creating the organized video game speedrunning community. The game included a built-in feature to record and replay gameplay as lightweight demo files that could be shared easily over internet bulletin boards. This infrastructure allowed players to compete globally. The Doom speedrunning community has remained active for decades, with runners still breaking records in 2019 that had originally been set in 1998.