There are many different APIs available in Drupal. These are some of the more essential APIs:
- Plugin API
- Block API
- Routing System
- Configuration API
- State API
- Database API
- Entity API
- Form API
- Logging API
- Menu API
- Render API
- Services and Dependency Injection
- Events
- Update API
Consult Drupal 8 APIs for a more detailed list.