Doom was the game responsible for the explosion in popularity of the FPS genre. Doom was so revolutionary and influential that, for a generation, games in the First Person Shooter genre were known merely as "Doom clones".
There are three different classes available in the game (warrior, cleric, and mage), each wit their own unique abilities. This gives more choice to the player over other DOS FPS games, which allows for more replayability and overall choice in gameplay.
A heretic that has descended to hell in order to destroy demons is about the gist of it, not much exposition is given, if at all. While it does not detract from the gameplay, there is little reason to play this game for the story.