Massively multiplayer online games (MMOGs) are growing in popularity, with the market in the West now worth more than $1bn (£511m). But which are the most popular? These are the five MMOGs with the highest number of subscribers worldwide, according to MMOGchart.com.
WORLD OF WARCRAFT
World of Warcraft is the world's biggest MMOG
Since launching in the US in 2004, Blizzard Entertainment's World of Warcraft has become the world's most popular MMOG.
It is available in seven languages and has more than 8 million subscribers.
The game takes place in the world of Azeroth where players can choose from a range of races and classes as their in-game character, or avatar.
Players perform quests within the game world to earn money, items and experience which improves their skill and power levels.
While early game quests can be completed by a single player, more advanced quests require players to work together in teams of up to five.
These kind of team activities take place in "instance dungeons", multiple copies of in-game locations where players can carry out quests without other players intruding.
Lineage was the first MMOG from South Korean publisher NCsoft, released in 1998.
The game is a medieval fantasy based on a series of South Korean comic books which follow the quest by a dispossessed prince to reclaim his throne.
Lineage has enjoyed huge popularity in South Korea and other Asian countries, with more than three million subscribers worldwide, according to NCsoft.
Dragons can be tamed in Lineage II
The sequel to Lineage was released in South Korea in 2003 and has more than one million subscribers.
Lineage II expands on its predecessor's "taming" system. which allows players to domesticate non-player characters (NPC) animals such as dogs, wolves and dragons, and to raise them as pets.
NCsoft releases regular game expansion packs, or "chronicles", free-to-subscriber downloads which introduce new storylines, quests and game areas.
RuneScape from UK developer Jagex has nearly one million subscribers, although a free version of the game is available, supported by in-game advertising.
The game was created by brothers Andrew and Paul Gower and released to the public in 2001, originally operating out a bedroom in their parents' home.
The game is set in the fantasy world of Gielinor which is inhabited by players and NPCs.
Players work their way through increasingly difficult quests to improve their skills levels and earn rewards.
FINAL FANTASY XI
The character Cid appears in many Final Fantasy games
Final Fantasy XI (FFXI) is an online instalment of the multimillion-selling Final Fantasy franchise.
It was released for PC and PlayStation2 (PS2) platforms in Japan in 2002.
FFXI is the first MMOG to support simultaneous multiplayer gaming across different platforms, meaning players using PC, PS2 and Xbox 360 can all play together online in the same game.
More than 500,000 people have subscribed to the game, according to its publisher Square Enix.