The NPAPI is a cross-platform plain-C plugin interface used by several web browsers to allow custom native-code to handle certain content types. It is used by Firefox, Chrome, Konqueror, and Opera, among others. When a browser encounters a content type that a plugin wants to handle, it loads that plugin, allocates a region of the web page for the plugin to control and then streams data to it so that the plugin can render the data as it sees fit.
Adobe’s Acrobat, Shockwave, Flash renderers are examples of plugins built against this plugin API.
The following web page provides various resources for using the NPAPI: