One of the aspects of the new object oriented architecture in Drupal 8 is the introduction of services. Services can be provided by any module and provide a mechanism for exposing reusable functionality by way of interfaces and classes. All services are instantiated via the container which is responsible for injecting a service’s dependencies.

Since services implement interfaces and are always instantiated via the container, we have the opportunity to alter what the container returns, ultimately allowing us to swap any existing service with a new one.