Before discussing about ajax let’s discuss Synchronous & Asynchronous first.
We can send one request at one time and have to wait for the response before send the second request. i.e. It blocks the client browser until operation completes.
We can send other request before getting the response of first request. i.e. It does not blocks the client browser.
Before ajax, synchronous requests are used by web applications. It means when we hit submit, application will open a new page with new information. With ajax, an application can send the request, get the response and update the same page.
Ajax refers to asynchronous java script and xml. It is not a not a programming language. Ajax is of Asynchronous type. It normally uses XML, plain text or JSON to communicate with server i.e. for data transfer.
Note: Ajax is technology independent.