Where it all started.
You are either a Terrorist or a Counter-Terrorist, pretty self explanitory there. If you don't know what a terrorist is though, you need to go back to elementary school. If you are a terrorist you try and set a bomb at a specific spot in a level, or guard hostages you have captured, sounds easy right...wrong. This is a multi-player game, so you have people with reflexes and experience with the game to destroy you...until the next round. As a Counter-Terrorist you are to do the opposite of what the Terrorists do, you have to disarm a bomb they have planted or guard the bomb if you've killed the carrier, and rescue the hostages they've captured. Or, you can do what this game is meant for...just flat out killing everybody in a team deathmatch.
With all of the killing you'll be doing you have to choose the right weapon right? Yes, Counter-Strike is not short on any kind of firearm to blast your enemy away with. Although the developers thankfully did not include rocket launchers or grenade launchers, because that just wouldn't be fair at all. However they did include a nice hefty machine gun packing 100 bullets per clip, allowing no one to escape as you mow them down. Along with the massacre weapon there are the more, shall I say, traditional weapons. Five types of hand guns, three types of sniper rifles semi-automatic and automatic, with about three or four semi-automatic rifles. Not to mention two shotguns, which you probably won't be using, and three SMGs. Nice as all this is it cannot beat the knife, the ultimate slap in the face to be killed by.
Hopefully you will at least give Counter-Strike a try, if not this one Source or Condition Zero. You WILL have fun with this game if your looking for a nice competative FPS.
Side-note: Counter-Strike 1.6 has the most mods of any of the games out, with the most custom maps, since this game has been out for the longest its what you should expect. Happy hunting.