The concept of an object that manages web service requests from script within a web page originated several years ago with Internet Explorer 5.0. The original object was an ActiveX object, and Microsoft continued to support that approach with IE 6.x. Now, though, like the other browsers, Microsoft supports the
XMLHttpRequest object is one of the simpler browser objects to work with: a web service request is created, the request is sent, and a callback function is invoked to process the response. Rather than all of this activity spread across many pages, as is the case with traditional web applications, it's all accomplished within the same web page, without any page reloads.
There are so many acronyms in this business. I use
XMLHttpRequest throughout much of the book, but I, and other developers, also sometimes just say XHR to save both time and typing.