In Counter Strike, the game is made up of rounds. Before the start of each round you have a few seconds to buy any firearms you feel may be necessary to winning the round. Your team (Counter Terrorist or Terrorist) can win the round by either completing their objective or by eliminating the other team. In the game there are three different scenarios you can play in.

Bomb/Defuse

This is the most common and popular type of game played in CS 1.6. Counter terrorists can win by defusing a planted bomb or eliminating the opposition. Terrorists win by bombing a target (Planting C4 in certain areas of the map and waiting for it to explode) or eliminating the opposing team.

Hostage Rescue

A less popular (but still fairly active) gameplay mode. As the title would suggest, the  counter terrorists must rescue  the hostages. If you're playing as a terrorist, you must simply prevent the counter terrorists from doing the above. Like all the scenarios, you can also win the round by eliminating the opposing team.

VIP

A very unpopular mode of gameplay amongst CS players, due to the fact that this mode of gameplay is very simple for terrorists, and very difficult for counter terrorists. The CT's win by ensuring the VIP (Chosen at random among the team members) escapes to the "safe point". In order for the Terrorists to win, all they must do is eliminate the VIP.