What is Ajax, and Does it Matter?

In My Humble Opinion... (IMHO): What is Ajax, and Does it Matter?:

"Ajax is composed of the following web technologies:

* HTML CSS: displays the information in the browser.
* JavaScript: dynamically displays content, and allows the user to interact with it.
* XML XSLT: Using the browsers XMLHttpRequest function data can be requested, sent, or changed on the Web server."