Ajax is an acronym that stands for ‘Asynchronous Javascript And Xml’. It is simply a means of loading data from the server to the web browser without reloading the whole webpage.
Ajax makes use of the JavaScript-based XMLHttpRequest object to send and receive information to and from a web server asynchronously, in the background, without interfering with the user's experience.
Ajax has become so important that most application that makes use of Ajax to some extent. Examples of some large-scale Ajax-driven online applications include Gmail, Google Docs, Flickr, Google Maps, Facebook, YouTube, and so on.
You must understand that different browsers implement Ajax differently, meaning that if you are adopting the typical JavaScript way to implement Ajax you have to write the different code for different browsers to ensure that Ajax would work across browsers.
However, jQuery simplifies the process of implementing Ajax by taking care of those differences existing amongst browsers. jQuery offers easy methods such as $.post(), load(), $.get(),and so on, to implement the Ajax that works seamlessly across all the browsers.