Counter-strike uses the same engine as Half-Life, There are several diffrent versions of Counter-Strike, Counter-Strike: Source, Counter-Strike: Condition Zero and Counter-Strike 1.6
Counter-Stike 1.6 Edit
Counter-Stike 1.6 is a FPS at its pureist. Counter-strike 1.6 is made for online, where Terrorists and Counter-Terroists fight it out in battle arena's.
Counter-Strike: Condition Zero Edit
Same thing as Counter-Stike 1.6 exept for a small graphical update and Bots.
Counter-Strike: Source Edit
Counter-Strike source features A new physics engine, With the new engine there are alot of new Features Including Smart AI bots, alot more realism and alot more!