Doom, one of the first major PC FPS and the reason why video games exist. Okay, that's not true, or maybe just a little bit true. Doom is the story of a man working on Mars when an experiment goes awry and a gateway to Hell literally opens. It's Doomguy's job to kill all of these spawns of Satan and to dive into Hell itself to show these monsters what Earth's really made of. He'll do all of this with an arsenal of insane weapons and an eye for picking up ammo and keycards from the labyrinthine halls on Mars and in Hell.
The gameplay by modern standards is very simple. Aim the gun at what you want to shoot and shoot. Enemy AI isn't at all intelligent, so demon heads and robot spiders and all of Hell's minions will generally just charge at the player or shoot whatever weapon they have from an often annoying perch.
The game is broken up into several acts, each of which contain several stories. The acts are generally just a change of scenery, showing how Doomguy is progressing across Mars and into Hell. The levels are just huge, entirely explorable levels filled with long halls, platforms raised with buttons, and keycards. Also, lots of darkness. Basically, the player will have to search through and backtrack through each level hunting down the keycard to allow him access to continue progressing.
Doom is and always will be a PC game, though its been ported to several platforms since. It's also spawned a number of sequels and even a major Hollywood movie. Don't watch that though. Just play the game.
 Hardware Requirements
386 processor operating at a minimum of 33MHz or any Pentium® /Athlon® processors
Windows 95/98/ME/2000 operating system
8 MB RAM
20MB of uncompressed hard disk space
100MB of free hard drive space for the Windows swap file
A 100% Windows® 95/98/ME/2000-compatible computer system including compatible 32-bit drivers for video card, sound card and input devices
A 100% Windows® 95/98/ME/2000-compatible true 16-bit sound card and drivers
100% Windows® 95/98/ME/2000-compatible mouse and driver
100% Windows® 95/98/ME/2000-compatible keyboard