Drupal 8 introduces many new APIs and keeps only some of the Drupal 7/6 API. This section details the APIs in Drupal 8.
Before diving into the documentation below you might want to read the background and prerequisites which explains some concepts and terminology that will be assumed knowledge in much of the developer documentation.
For a crash course, but with some real depth, check out Understanding Drupal 8. It provides an overview of many of the key D8 APIs.