AJAX is not a new programming language, but a new way to use existing standards. AJAX is the art of exchanging data with a server, and updating parts of a web page - without reloading the whole page.
Ajax is a group of interrelated web development techniques used on the client-side to create asynchronous web applications. With Ajax, web applications can send data to, and retrieve data from, a server asynchronously (in the background) without interfering with the display and behavior of the existing page. Data can be retrieved using the XMLHttpRequest object. Despite the name, the use of XML is not required (JSON is often used instead. See AJAJ), and the requests do not need to be asynchronous.
Representational state transfer (REST) API: Representational state transfer (REST) is an architectural style consisting of a coordinated set of constraints applied to components, connectors, and data elements, within a distributed hypermedia system.