Drupal is an open source content management system. It is distributed under the GNU license.
Since Drupal is an open source, it means that we can look at the code very easily. Unlike other Content Management tools which uses all the new hard to understand technologies like Web Services, Java etc, Drupal uses PHP which is easy to use and learn. This makes the code easy to understand and modify as per your needs.
There are tons of APIs that come from Drupal. This app provides a listing of all the Form APIs and other APIs. Moreover, this app provides an easy to use search interface to do a quick search for the desired form API.
Continue to look for updates to this app, as we add new APIs.
The API for this app was obtained from drupal.org website. The API on the most part is backward compatible, but the source of this API from drupal.org was for Drupal 7.