REST APIs

REST is an architectural style that uses simple HTTP calls for inter-machine communication instead of more complex options like CORBA, RPC, or even SOAP. Using REST means your calls will be message-based and reliant on the HTTP standard to describe these messages. Using the HTTP protocol means REST is a simple request/response mechanism. Each request returns a subsequent response.
It works because you’re not tying your API to your client-side technology. One could imagine that this API is accessible from a client-side Web project, an iOS app, an IoT device and even a Windows Phone.
Rest states  “representational state transfer”, is an architectural style consisting of a coordinated set of components, connectors, and data elements within a distributed hypermedia  system, where the focus is on component roles and a specific set of interactions between data elements rather than implementation details. REST API is providing various URIs to client side technology (Front end technology) from where client side technology fetches the data.  REST APIs is the internet (web) way on how you do ‘stuff’ on your ‘things’ providing/obtaining the required ’information’ in the process. In other words, it is the web language that allows you to perform operations on resources providing details in the ‘request’ and getting back details in the response.