First time you enter into Internet you realise another leisure dimension has begun for you. You were used to play alone at home or maybe with one or two friends using LAN, but now you can play online with dozens of players all around the world. But online multiplayer games can be divided in two main categories: free games and pay per play games.
First there were many free games, but some years ago pay per play games began to become notorious, since now that many multiplayer games are not free. You can distinguish between web based games and those ones where downloading a client is needed. Generally, web based games are free, and client based games have several options. Web based games don't have amazing graphics, and are generally turn based games, like swcombine or Hattrick. Pay per play games have their own graphic interface, because they are not web based and have their own client, so graphic are much better, taking advantage of hardware potencial.
So free games are simpler but not less addictives. We also have those games which are not free and you have to purchase them, but its multiplayer modes are free, like Unreal Tourneament 2004 or Half-Life 2. And finally you have those ones called MMOGs which you must pay for your copy or cdkey and also pay for a subscription for playing at them.