Plugins are groupings of code that are imported into and included by a host application. This is actually a subclass of Application with some overrides that make it suitable for mounting within an app. Other than that, it's basically the same thing as an Application, you can add routes/initializers to it and even include other plugins. Routes are not actually added into the application until the user mounts the plugin with a call to mount("some/path", YourPlugin) in the routing DSL.