What does AJAX mean?

Ajax is an abbreviation of Asynchronous Javascript and XML. It is used for creating better, faster, and more interactive web applications. Ajax is a set of web development techniques that involve XHTML, CSS, Java Script, XML, and XSLT. It acts as a means of communication between a web page and a server and that allows for asynchronous data retrieval without having to reload the page a visitor is on.
Here is an example of some web applications where the Ajax used:

Autocomplete Search Form
If you use Google to search any term or keyword, then you may know that while typing any keystroke in google search you see a drop-down window showing predicted autocomplete words related to your typed keystroke.

ajax autocomplete result in google search

Actually, when typing into the Google search bar, it starts to use Ajax to get common results from the database on each keystroke.

Login Forms
Ajax can authenticate user’s credentials with MySQL database without reloading the whole page. If the user is registered then redirect the user to a home page otherwise display an error with a message “wrong password”.