Hi,
i don't know Ajax.give some idea to Ajax.

Ever hear of the all fancy "Web 2.0"? AJAX is pretty much what defines this (besides glossy graphics). But basically AJAX is just the use of JavaScript and any server side language to be able to process data without refreshing the page. Well a new page is virtually made and processed but the user never sees this. Just the output which is then returned back to JavaScript to be used by the document. Here's how the average AJAX request goes threw - JavaScript(send)->PHP(send)->JavaScript(receive and use).
http://www.w3schools.com/ajax
That is an example of Ajax using JavaScript and ASP. You can use JavaScript and PHP, etc. As stated before.

w3 might be a bit too uptight and technical if you have no idea at all here is a link to a decently good explaination on wikipedia
http://en.wikipedia.org/wiki/Ajax_(programming)

For a LESS formal explaination try about.com at this link
http://webdesign.about.com/od/ajax/a/aa101705.htm

If your thinking about learning it i would reccomend this book from sitepoint
http://www.sitepoint.com/books/ajax1/?nocache=1099150945
I havn't used it personally but i have heard good words about it and it is fairly cheap