Single Player Games
editSome single-player games contain virtual worlds populated by non-player characters (NPC). Many of these allow you to save the current state of this world instance to allow stopping and restarting the virtual world at a later date. (This can be done with some multiplayer environments as well.)
A recent example of a Virtual World within a single player game is Minecraft[1]
Minecraft can be played either as single player game, or online multiplayer, players have the option of using either.
Minecraft players can use User-generated content[2] to create their own Minecraft world, some will create a Mirror world[3] which is described as "a representation of the real world in a digital form". As well as mirrored worlds, users can create anything they wish, e.g. a mansion or a castle. All of which creates their own personal experience.
Single-player games such as Minecraft allow you to optionally create your own world without other players, and then combine your skills from the game to work together with other players and create bigger and more intricate environments. These environments can then be accessed by other players, if the server is available to other players then they may be able to modify parts of it, such as the structure of the environment.