Counter strike is an online-only first person shooter game. There isn't a main game, but you instead join a server and pick a team (counter terorists or terorists) and compete to complete a mission in rounds. After the round is over, you repeat the mission again.
Counter terorist need to save all the hostages, defuse the bomb, or kill the enemy team.
Terorists you have to protect the hostages until the round is over, plant the bomb, or kill the enemy team.