Persistent browser-based game
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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 typically allows dynamic system modelling elements to develop and progress (stuff happens), even while the player is offline. Such games often last for several months.