In old days, web development was talking about Javascripts which is good for implementing client-side validations. This helps saving lot of time on customer end to wait for server's response and also saves on server's load & bandwidth. With developing technologies, AJAX is something which helps a lot.
AJAX is a group of inter-related web development techniques used for creating interactive web applications. It increases responsiveness and interactivity of web pages achieved by exchanging small amounts of data with the server "behind the scenes" so that entire web pages do not have to be reloaded each time there is a need to fetch data from the server.