Ajax is a new technology that is being used in many sites now a day to improve the richness on the client side. Some examples like when you compose a mail in Gmail or in Yahoo mail, in the TO address bar, if you type some letter (say a or b) then email address that starts with a or b that is stored in your address book gets displayed below the address bar.
This is nothing but a Asynchronous call to the Database and updating the DOM(Document Object Model) of the HTML page. So no need to refresh the page and to load all the Data.
and to answer what you asked in the subject of this thread, No, AJAX is not a new technology or say a "discovery of year 2005". But yeah credit goes to Google, Yahoo for making it's presence felt recently in the technology world due to Gmail, yahoo mail, google earth etc.
To also add that this is not a new technology and it really is not a technology but a new way to approach a web application. IE has had this in its browser for many versions and you can approach this same style of programming with iframes and pop up windows. The only thing that the XMLHttpRequest object gives us is a more reliable means to tranfer the information from the server layer to the client layer.
I guess AJAX is not really a new technology. Its just a different way of implementation.
Not that I am MS technology fan. Microsoft-ASP had this XMLHTTPObject and using MSXmlParser We used to have similar stuff being done few years back also. post request using this and show in differnt IFrames.might be done with JSP also. I feel AJAX has changed the way we can handle validation on forms.