REST (Representational state transfer) – is a software architectural style that defines a set of constraints to be used for creating Web services. Web services that conform to the REST architectural style, called RESTful Web services, provide interoperability between computer systems on the Internet. RESTful Web services allow the requesting systems to access and manipulate textual representations of Web resources by using a uniform and predefined set of stateless operations1.
For example, let us look at how Web applications behave:
a network of web pages (a virtual state-machine), where the user progresses through an application by selecting links (state transitions), resulting in the next page being transferred to the user and rendered for their use (representing the next state of the application).
Because Cisco ACI is based on a hierarchical object model EVERYTHING is represented as an object and every object can be manipulated via REST, using RESTful API calls.