With AJAX you can send and receive information to and from server on a small parts. You don’t need to get entire web page with new content. AJAX gives you ability to get exact this small part of information which is requested by the client and to put this information in exact place where the information stands for.
In classical Client-Server web application development, user have to fill some web forms, and click on Submit button. Then you have to collect user-data and post this information to the server. Next you have to get some data from a database, put this data in new web page and return this web page to user which looks to user like page-reload. This is old and slower.