Webmachine handles the HTTP protocol for Zotonic. Webmachine is made by Basho.
Webmachine is not much like the Web frameworks you're used to. You can call Webmachine a REST toolkit if you like, and we won't argue with you.
It is an application layer that adds HTTP semantic awareness on top of the excellent bit-pushing and HTTP syntax-management provided by mochiweb, and provides a simple and clean way to connect that to your application's behavior.
A Webmachine application is a set of resources, each of which is a set of functions over the state of the resource. We really mean functions here, not object-methods, infinite-server-loops, or any other such construction. This aspect of Webmachine is one of the reasons why Webmachine applications are relatively easy to understand and extend.
Any Zotonic module can add resources. Dispatch rules defined by the module map the URL query path to the correct resource.