There's two versions of this game (also of Blue Shift and Opposing Force) out, one uses the original GoldSrc engine, and another which was ported to the newer Source engine with very few (barely notable) changes to how the game appears or works.
Also in the original release/GoldSrc (called "Half-Life") both singleplayer and multiplayer are included, but in the Source re-release they are split in two different titles, one for singleplayer (Half-Life: Source) and another for multiplayer (Half-Life Deathmatch: Source)
