With any web application there will need to be requests made to the web server to get the latest versions of dynamic data. In an application like bttlxeMail Webmail every little action requires data to be retrieved from the server. Selecting a message, flagging a message, checking for new e-mail, managing contacts, notes, calendar, meeting requests, etc - all of that data needs to be retrieved from the server. Typically each request would be sent back to the web server and a new HTML page returned to the user’s browser with the updated content. For a data-intensive system like ours though such an approach would introduce a serious performance problem, both putting stress of the server and causing long delays for the user.
In basic terms, AJAX allows us to make a request to the server for data, retrieve the response and process it without reloading the web page. Look <font size="2">here</font>  for more information.
Read full article at http://weblogs.bttlxe.com/bttlxeMail/archive/2005/07/27/150.aspx