This document discusses dependency injection (DI) in AngularJS. It explains that DI allows high-level modules to depend on low-level abstractions rather than implementations. It then covers the different ways to define providers in AngularJS, including using the $provide service, as well as factories, services, values, and constants. It also discusses decorators as another way to modify existing services. The document provides recipes for when to use each provider type and concludes with some additional facts about injectors and ensuring scripts are in the proper order.