Half-Life: Counter-Strike is the ultimate multiplayer gaming experience, combining all the elements of the Half-Life online universe with new single and multiplayer content. At the heart of this package is Counter-Strike, the number one played online action game built as an add-on for Half-Life, the PC CD-ROM named Game of the Year by over 50 publications worldwide.
Counter Strike started out as all other Half Life mods, as a fan created modification. Over time Counter Strike gained popularity via the Internet and has become the most widely played online action game. It was even release as an official retail add on for Half Life. Counter Strike however is still available for free download. You must own and have the original Half Life installed in order to play this version of Counter Strike.
The retail version of Counter Strike includes other online multi player action such as Team Fortress and Opposing Force, plus some manuals and documentation that aren't included. You do not need to own Half Life if you purchase the retail version of Counter Strike.