The XMLHttpRequest Object specification defines an API that provides scripted client functionality for transferring data between a client and a server.

Documenting changes since Last Call, the Web API Working Group has released an updated Working Draft of “The XMLHttpRequest Object.” The core component of Ajax, the XMLHttpRequest object is an interface that allows scripts to perform HTTP client functions, such as submitting form data or loading data from a remote Web site. Read about the Rich Web Clients Activity.

Read more about AJAX programming from Wikipedia and Rich Web Clients page at W3C.