Answer 1

AJAX typically involves sending HTTP requests from client to server and processing the server's response, without reloading the entire page. (Asynchronously).
If you are totally new to AJAX (which stands for Asynchronous Javascript And XML), the AJAX entry on wikipedia is a good starting point:
Like DHTML and LAMP, AJAX is not a technology in itself, but a group of technologies. AJAX uses a combination of:
HTML and CSS for marking up and styling information.The DOM accessed with JavaScript to dynamically display and interact with the information presented.A method for exchanging data asynchronously between browser and server, thereby avoiding page reloads. The XMLHttpRequest (XHR) object is usually used, but sometimes an IFrame object or a dynamically added tag is used instead.A format for the data sent to the browser. Common formats include XML, pre-formatted HTML, plain text, and JavaScript Object Notation (JSON). This data could be created dynamically by some form of server-side scripting.