This guide covers everything from tutorials and best practices to working with JavaScript and the Drupal API. You can also view a searchable API reference at http://api.drupal.org.
You are strongly encouraged to contribute code back to the community. For more information, see the Contribute code section of the Getting Involved guide. There you will find detailed information on submitting patches and maintaining your own project.
For information on upgrading and backwards compatibility, see On backward compatibility: the drop is always moving.