JSON-RPC is a stateless, light-weight remote procedure call (RPC) protocol.

This module implements a framework for creating non-RESTful services. It is intended to be the platform on which a decoupled Drupal administration and introspection interface can be built.

While JSON API module handles entity data and REST can be used to create custom REST resources, some Drupal actions can’t be neatly handled with REST principles. Clearing cache is a perfect example.

Source: https://drupal.org/project/project_module/feed/all