An American online multiplayer first person shooter first developed as a mod for Valve Software's Half-Life before they acquired the IP and made it into its own retail game. Often called Counter-Strike 1.6 to differentiate from the series.
The game pits terrorists and counter-terrorists against each other using tactics and strategy to see who wins, either by completing the objective (bomb defusal or hostage rescue), running the time out or killing their opposing team.
This tag implicates counter-strike_(series) (learn more).