Asynchronous JavaScript and XML (AJAX) is a method of communication between a client and the server where the website resides. AJAX makes it possible to dynamically respond to actions on the client side. For example, it is possible to directly check an e-mail address while you are typing! Or set a color of an object when clicking on another object. Ajax is most used to dynamically fetch data from the server to use it on the web page.
Until APEX 4.0, using Ajax in your web page required advanced programming with JavaScript. But since APEX 4.0, it is a lot easier to use AJAX with the help of dynamic actions. We will make a page with a tag cloud and a region where you can see the news articles. When you click on a tag in the tag cloud, APEX will show the articles that are related to the selected tag. This information is an AJAX call and is implemented using a dynamic action.