Many online games, particularly MMORPGs, require a monthly payment to pay. This fee goes towards upkeep for the game, such as maintaining servers or the salaries of customer support,etc.
With Pay-to-Play, the gamer pays a monthly fee for upkeep of a game or service. Massive Multiplayer Online Roleplaying Games usually require a monthly fee to play. Normally ranging from $5 to $15, this money is used to maintain the servers and pay developers to create new experiences for the game world. Online gaming services such as Xbox Live sometimes require a monthly fee as well, which covers the ability for the whole service and online gameplay for numerous games as a whole.
Some MMORPGs and Game services have been free, but with downsides usually. For MMORPGs, they usually have ads. In the case of Guild Wars, there is no monthly fee, but the cost of continuing to play new experiences is buying expansions, but this gives players the option to play one game without a fee.
The PS3 has a free online service as an alternative to a paid Xbox Live service. It is more limited than Xbox live in functionality in some ways, but it is unclear if that is because it is not as establishe as XBOX Live or because the there are no fees. Gamers differ on which service they prefer. Many Xbox Live users want the service to go free, but it is unknown what the implications for the service would be.