What happens when the user submits a request?

When user submits request,the steps below occur sequentially: 

1.  Web browser requests for the content of just the part of the page that it needs.

2. Web server analyzes the received request and builds up an XML message which is then sent back to the web browser.

3. After the web browser  receives the XML message,it parses the message in order to update the content of that part of the page.

AJAX uses JavaScript language through HTTP protocol to send/receive XML messages asynchronously to/from web server.

Asynchronously means that data is not sent in a sequence.