Half-Life is a first-person shooter developed by Valve Corporation and published by Sierra Entertainment in 1998. In this game, you take the role of the theoretical physicist Gordon Freeman, who must fight his way out of the Black Mesa Research Facility after an experiment went horribly wrong.
This game has several expansion packs and mods, most notably Half-Life: Opposing Force and Half-Life: Blue Shift. Opposing Force takes place in the same storyline, but from the perspective of one of the marines in the Hazardous Environment Combat Unit sent to cover up the incident. Blue Shift is also there, but this time as the security guard seen at the door you passed at the beginning of the original game.
In this video, you can see Half-Life: Uplink, one of two demos, with maps and a storyline not found in the full version. The other demo, Half-Life: Day One, contained the first fifth of the full game and was only shipped with certain video cards of the time.