The reason Counter-Strike is so popular is because it's simple yet addictive to play
If you don't already know, Counter-Strike is a simple fps shooter that pits terrorists against counter terrorists in a vast selection of maps. Some small objectives are used, but the main aim is to kill the entire other team. As with all online team games, you have real people fighting along side you, but what makes Counter-Strike stand out is that you really have to work as a team to win.
What makes this really great, is that everything is so simply designed, that you can pick it up and play it whenever you want. The controls are really easy to use, and you can learn the objectives and different maps in just minutes!
The introduction of the new source engine has also meant that graphically and realistically, this game really shines. The so called "rag doll" effects make bodies droop over when they die. Lighting and shading are also the top of its class ,and the shadow effects are really quite something.
In total, this game is quite simply one of the best. Half -Life 2, the game it comes with is even better, so get it now!