Overview#Hypermedia As The Engine Of Application State is a constraint of the REST application architecture that distinguishes it from most other network application architectures.
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 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.