Blocks in Drupal 8 are plugins. However, the blocks you create in the UI are content entities and the placement of both in the block layout are configuration entities. So, the block system is a good example of how entities and plugins work hand in hand in Drupal 8. We will talk in more detail about plugin types and entities later in the book.
The block system in Drupal 8 is a great shift from its predecessor. Before, you had to implement two obligatory hooks plus two optional hooks if you wanted the block to have a configuration, and the latter was always saved somewhere that had nothing to do with the block itself. In Drupal 8, we work with a simple plugin class that can be made container-aware (that is, we can inject dependencies into it) and we can store configuration in a logical fashion.
So, how do we create a custom block plugin? All we need is one class, placed in...
