The principle is that a client interacts with a network application entirely through hypermedia provided dynamically by application servers. A REST client needs no prior knowledge about how to interact with any particular application or server beyond a generic understanding of hypermedia. By contrast, in some Service-oriented Architectures (SOA), clients and servers interact through a fixed interface shared through documentation, agreed upon schema or an Interface Description Language (IDL).
Hypermedia As The Engine Of Application State (HATEOAS) constraint decouples client and server in a way that allows the server functionality to evolve independently.
Hypermedia As The Engine Of Application State requires some form of Web Linking or Link Relation Types for a client to be able interact with a network application entirely through hypermedia provided dynamically by application servers.