The Authentication API is new in Drupal 8.
Blocks API in Drupal 8
The CKEditor API is new in Drupal 8.
Documentation for Cache API in Drupal 8.
Configuration API in Drupal 8.
Drupal 8 Entity API documentation.
Entity Validation API in Drupal 8.
The Filter API is conceptually the same in Drupal 8 as it was in Drupal 7 and before. The major changes are:
Form API in Drupal 8.
In Drupal 8 watchdog has been refactored to an OO, PSR-3 compatible framework.
Menu API in Drupal 8.
Drupal 8 has a Middleware API that it adopted from the broader PHP community: http://stackphp.com/.
Migrate API in Drupal 8.
Plugin API in Drupal 8.
The Quick Edit API is new in Drupal 8. It is inspired, and in some parts functionally identical to the Drupal 7 Quick Edit module.
RESTful Web Services API in Drupal 8.
Render API in Drupal 8.
Responses in Drupal 8
Routing system in Drupal 8.
Serialization API in Drupal 8.
Services and dependency injection in Drupal 8.
The State API provides a place for developers to store information about the system's state. State information differs from configuration in
The Text Editor API is new in Drupal 8. It is inspired, and in some parts functionally identical to the Drupal 7 Wysiwyg module.
You can create step by step guides about the user interface for the Drupal 8 Tour module by adding a YAML document to your module's config
When you include text in code, and the text will be displayed in the Drupal user interface, it needs to be internationalized (prepared
Typed Data API in Drupal 8.
Update API for Drupal 8 (hook_update_N() etc.)