CS is Counter-Strike a mod for the game Half-life. Basically in the game you choose to be either a terrorist or counter-terrorist (or VIP in required maps) and win the rounds by either killing the opposing team or completing the mission. Eg, in cs maps (cs_assualt, cs_office) the mission for the ct's (counter-terrorists) would be to rescure the hostages, and for the t's (terrorists) to guard the hostages, or win the round by killing the opposiing team.
In de maps (de_aztec, de_dust) the object for the terrorists here would be to plant a bomb at any of 2 bombsites and for it to detonate, and object for ct's woulf be to defuse the bomb before it explodes or agian kill the opposing team.
You can buy plenty of guns, pistols, rifles, equipment etc.
Fun to play with your friends and tk them, ahahha!