If you look up first-person shooter in the Dictionary, Doom will be the definition.
You are a former Marine sent to Mars working for the Union Aerospace Corporation, a company experimenting in teleportation technology. An accident opens a rift between our universe and hell, releasing demons and other creatures onto Mars. The company cannot withstand the onslaught of the horde of evil, and is quickly dispatched. Your job is to stay alive and get out.
Graphics: The graphics were the gold-standard of the time, and is one of the first games that inspired PC owners to upgrade their computers. The artwork was well crafted, the textures impressive, level layouts thoughtful and immense, and everything blended seamlessly. The details really get to the player, such as pentagrams on the floor, corpses impaled on pikes and twitching, and other representative image of Hell that have imposed themselves on our own dimension are everywhere. Even today the artwork impresses, even given the limitations of the PCs at the time of Doom's initial release.
Sound: The sound took full advantage of dedicated sound cards that had become almost standard in PCs by 1993. Enemies hiss, spit, claw, and bit with disturbingly accurate audio. The shotgun and chainsaw are satisfyingly visceral. In fact, many of the sound bites used for the original Doom have been appropriated for hundreds of other forms of entertainment due to their iconic nature. The music is okay, but more of an afterthought. It is moody without being intrusive, however, which is an accomplishment in and of itself.
Gameplay: Doom plays fantastic. The pacing is perfect. One moment you are unaware whether there is an upcoming enemy in the silence, the next there is an onslaught for ten minutes of neverending shotgun blasting. Doom also introduced multiplayer gaming to FPS, one of its primary attractors that would go on to become standard for most games released today. Only a few games are single-player today, though many include single-player campaigns. Finally, Doom was heavily modified by independent programmers with everything from custom sound effects from the Three Stooges to full conversions mimicing the Ghostbusters. The "WAD" files that allowed for the full conversion inspired the game modding community today.
Doom is an iconic and historic game that has inspired hundreds of successors. It is a must-play for anyone who enjoys video games in general.