Persistent browser-based game
A persistent browser-based game (PBBG) is a computer game that is both browser-based (accessed and played over the Internet only through a web browser) and persistent (able to progress with successive playing sessions).
PBBGs can provide the depth of experience and sustainability seen in some application-based games, but because they don't require any special software to play, they are more accessible and portable than such games.[1]
Persistent browser-based games usually rely on some kind of server-side code, such as Perl, PHP, Ruby, Python, or Java; though some will use technologies like Flash, ActiveX, and Java applets to store data on the client's computer. Games relying on client-side technology are rarer due to the security aspects that must be dealt with when reading and writing from a user's local file system - the web browser doesn't want web pages to be able to destroy the user's computer, and the game designer doesn't want the game files stored in an easily accessed place where the user can edit them.
For the more common games of this kind, the server-side code will store persistent information about players and possibly the game world in some kind of database, be it a flat text file, relational database, or game objects serialized from the server-side language to a binary file.
Sustainability
Sustainability, especially when combined with persistence is a key distinction of a PBBG. This allows dynamic system modelling elements to develop and progress (stuff happens), even while the player is offline. Such games often last for several months.