Generator commands delegate Rails::Generator::Base and implement a standard set of actions. Their behavior is defined by the way they respond to these actions: Create brings life; Destroy brings death; List passively observes.
Commands are invoked by replaying (or rewinding) the generator’s manifest of actions. See Rails::Generator::Manifest and Rails::Generator::Base#manifest method that generator subclasses are required to override.
Commands allows generators to "plug in" invocation behavior, which corresponds to the GoF Strategy pattern.