XMLHttpRequest class

Allows to interact with the servers
Attributes

readyState	the code successively changes value from 0 to 4 that means for "ready".
status	200 is OK
404	if the page is not found.
responseText	holds loaded data as a string of characters.
responseXml	holds an XML loaded file, DOM's method allows to extract data.
onreadystatechange	property that takes a function as value that is invoked when the readystatechange event is dispatched.

Methods

open(mode, url, boolean)	mode: type of request, GET or POST url: the location of the file, with a path. boolean: true (asynchronous) / false (synchronous). optionally, a login and a password may be added to arguments.
send("string")	null for a GET command.

taken from java.sun.com

Firstly, we need to know how to create an XMLHTTPRequest object. The process differs slightly depending on whether you are using Internet Explorer (5+) with ActiveX enabled, or a standards-compliant browser such as Mozilla Firefox.

With IE, the request looks like:

http = new ActiveXObject("Microsoft.XMLHTTP");

whereas in a standards-compliant browser we can instantiate the object directly:

http = new XMLHttpRequest();

Now we need to write some event handler which will be part of web page (user's page)

We can make our request of the server by using a GET method to an appropriate server-side script. Here's an example event handler called updateData which assumes that we have created our XMLHTTPRequest object (as above) and called it http:

function updateData(param) {
  var myurl = [here I insert the URL to my server script]; 

  http.open("GET", myurl + "?id=" + escape(param), true);
  http.onreadystatechange = useHttpResponse;
  http.send(null);
} 

Thirdly, then, we need to write a function useHttpResponse which will establish when the server has completed our request, and do something useful with the data it has returned:

function useHttpResponse() {
  if (http.readyState == 4) {
    var textout = http.responseText;
    document.write.textout;
  }
}

In this case, we have received our information as simple text via the responseText property of our XMLHTTPRequest object.