This document discusses hypermedia APIs and REST. It introduces the Richardson maturity model for REST APIs, which classifies them as level 1 (RPC), level 2 (RESTful), or level 3 (hypermedia). A level 3 hypermedia API uses hyperlinks to communicate available actions and business workflows to clients. This allows client-side behavior to be driven by links, making clients act like state machines. The document provides examples comparing how a book browsing/purchasing workflow would be implemented at levels 2 and 3, demonstrating how level 3 implies business rules and updates through links.