Counter Strike: Source help?

Just bought it off steam, and all the games I have gotten into are the game mode where you have 1 life per round and you have to save the hostages if you're CT and stop the CT if you're terrorists. However, I have seen gameplays on youtube where you have to plant a bomb and there are no hostages. Is this another add on I have to pay for and download, or is it part of the standard game and I just havent found a game of it yet?