CiviCRM 3.4.x and 4.0.x introduced API v3, a more consistent set of interfaces for integrating with CiviCRM using PHP, Smarty, Javascript, and REST. Building on this consistent core API, recent CiviCRM updates have introduced even more ways to manipulate your data -- such as chaining and CSV batch importing.
Thus, functions implemented according to the API v3 conventions can be invoked several different ways. If you would like to leverage this infrastructure for use with a new or customized API call, then download the latest release. With 3.4.6/4.0.6, external developers can expose API functions for new entities, new actions, and even generic actions.