Page "NPAPI" Paragraph 2

A plugin declares that it handles certain content types ( e. g. " audio / mp3 ") through exposed file information.

When the browser encounters such content type it loads the associated plugin, sets aside the space within the browser content for the plugin to render itself and then streams data to it.

The plugin is then responsible for rendering the data as it sees fit, be it visual, audio or otherwise.

So a plugin runs in-place within the page, as opposed to older browsers that had to launch an external application to handle unknown content types.