An extension is a plugin or extension for the API that allows one to add value to the functionality the API provides without jamming functionality into the core logic. An example extension might be to display information about the default view for an object. This information isn’t exactly core data to the object itself, but would be useful to know and be able to change via the API.
Extensions are multi-adapters that adapt a service object and a content object. Extensions are named adapters that are registered via an (IInterface, what is this called in human?).