The term AJAX stands for Asynchronous JavScript and XML, that is Asynchronous JavScript and XML. Web page loaded on a user's computer contains a program written in JavScript, which, depending on the user, generates a specific request and sends it to the Internet to the server that hosts the site. On the server, the request is processed and sent an answer that is accepted by the same program in JavScript. Requests and responses can be sent in a structured way, furnished on XML, which is a set of rules for storing structured data. In fact, often the requests and responses sent in any form, without the use of XML, but still referred to as technology used the term AJAX. The main thing is that the interaction with the server is in asynchronous mode, ie requests and responses are sent not with the change of Web pages, and separately, in the background. At the same time sometimes the user may not know that the query was executed, so as externally can all look exactly the same as in the simple execution of the script on the page.
To understand what this technology can be considered simple example of AJAX. When initially loading the page the user sees any text and images. Clicking on the link, it initiates sending a request to the server. Once a response is received, some elements of the page change while the page is not loaded and re-drawn, as with conventional technology. With AJAX you can create interactive Web pages, some of which change in response to user actions.