The term "Contrib" is used to refer to any package that is not just a simple plugin.
Foswiki contribs may
extend the functionality of Foswiki, in a lower-level way than plugins,
or provide alternative implementations for sections of the Foswiki core e.g. user management, or when an extension just can't be implemented as a plugin because it requires very close access to Foswiki internals,
or they might provide other files that Foswiki uses, for example language files,
or they might be packages of topics that implement a "Wiki Application", using the wiki macro language, TML.