From http://www.w3schools.com (Copyright Refsnes Data)

The XMLHttpRequest Object

With an XMLHttpRequest you can communicate with your server from inside a web page.

What is the XMLHttpRequest Object?

The XMLHttpRequest object is the developer’s dream, because you can:

When you type in the input box below, an HTTP request is sent to the server and name suggestions are returned from a name list:

Type a letter in the input box:

First Name

Suggestions:

Creating an XMLHttpRequest object is done with one single line of JavaScript.

In all modern browsers:

var xmlhttp=new XMLHttpRequest()

In older Microsoft browsers (IE 5 and 6):

var xmlhttp=new ActiveXObject("Microsoft.XMLHTTP")

In the next chapter, we will use the XMLHttpRequest object to retrieve XML information from a server.

The XMLHttpRequest object is supported in all modern browsers

The XMLHttpRequest object is not specified in any W3C recommendation.

However, the W3C DOM Level 3 "Load and Save" specification contains some similar functionality, but these are not implemented in any browsers yet.