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, and the Creating Drupal 8.x modules guides, both of which explain some concepts and terminology that will be assumed knowledge in the API documentation here.
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.
Finally, review the Drupal 8 backwards compatibility policy for an explanation of what may change between minor versions of Drupal 8 and what will remain stable.