Have you noticed how web pages have become more responsive lately? No more tedious waiting for a page to load after you have clicked its link. Instead, the page reacts instantaneously to your input. This is Ajax at work.
Like LAMP it is not a new technology, but a stack of existing ones. You cannot buy or download Ajax, but you can combine its composing technologies. Ajax uses these technologies to enhance the user experience on the web by bypassing the conventional request/response architecture of HTTP.