An add-on runs separately from the Foswiki scripts, e.g. for data import, export to static HTML, etc. Add-Ons normally do not call any Foswiki code directly, though may invoke Foswiki scripts. There are different types of add-ons, they may be stand alone scripts, browser plugins, office tool extensions, or even a set of topics that form a wiki application.
See other types of extensions:Contribs, Plugins, Skins