Modules are the building blocks of Zotonic.
Examples of modules are the /admin site, Atom feeds, the sitemap.xml, video embed code handling and SEO optimization. Modules also augment the functionality of other modules by adding extra templates and accompanying logic or adding handlers for internal Zotonic events. Good examples are the modules extending the mod_admin.
A module is a gen_server with accompanying templates, Webmachine resources, dispatch rules and more, all contained in a single directory.
The page Module structure has more information about the internals of a module.
Looking for more modules?
Check out the Zotonic Module Index, an index with user-contributed modules which are not part of the core Zotonic distribution.