The text below is from www.pbbg.org

What is a PBBG?

The PBBG definition is quite broad. The PBBG genre encompasses games of all traditional genres: role-playing, strategy, sports, simulation, and so forth.

PBBGs may be single-, multi-, or massively multi-player games. However, since an Internet connection is required to play, most developers incorporate massively multi-player and community elements into their PBBGs. A multi-player PBBG can be referred to as a MPBBG, but multi-player game play is usually assumed.

PBBGs may be text-based, or they may take advantage of browser plug-ins such as Flash or Java to create a more dynamic game play environment.

The PBBG definition is device-independent. In particular, a persistent game played within the web browser of a cell phone or other wireless device is still a PBBG. Such a game may be referred to as a mobile-enabled PBBG.

What is not a PBBG?

A PBBG is not the same thing as a MMOG (massively multi-player online game), generally speaking. There is some overlap, but MMOGs include games that are run as an application on your computer. Application-based games do not have the portability that PBBGs do, because you can only play them on a computer that has the game installed. Think of PBBGs as a subset of MMOGs -- those that are browser-based.

A PBBG is also not the same thing as a browser-based game, generally speaking. Browser-based games include arcade, puzzle, and card games which are over as soon as you move your browser to a different web page. These games lack the longevity of PBBGs, where progress is made over days, weeks, or months. PBBGs are a subset of browser-based games -- they are persistent browser-based games.