"Half-Life is a science fiction first-person shooter computer game series which consists of two full games, three episodic games, and three third-party made expansions, as well as a number of demos featuring exclusive content. The series has been ongoing since 1998. Most games and expansions in the series were developed by Valve Corporation and published by Sierra Studios, although some expansions were produced by Gearbox Software. Electronic Arts also took over distribution of the games in the series from Sierra from 2005 onwards. The series was initially released for the PC, but console ports of most installments have also been released.
The main protagonist of the Half-Life series is Gordon Freeman, a theoretical physicist and employee of the Black Mesa Research Facility. All expansion packs for the original game feature playable characters other than Freeman.
Source: Wikipedia, "Half-Life_(series)", available under the CC-BY-SA License.