XMLHttpRequest sends a request to a URL. The URL page can be a file at the server side, and may return data on the server for further processing. It's very important in some applications such as AJAX.

var req;
if (window.XMLHttpRequest){
req = new XMLHttpRequest();
}
else if (window.ActiveXObject){ //IE Browser
req = new ActiveXObject("Microsoft.XMLHTTP");
}

The get property of open() method requires some data from a URL page, and then process the return data.

req.open("GET", 'test.php?q=id', true);
req.onreadystatechange = handlereq;
req.send(null);
function handlereq()
{
if (req.readyState == 4)
{
var str = req.responseText;
document.write(str)
}
}

The put property of open() method sends data to a URL page, and do not need a response.

req.open("PUT", 'test.php?q=id', true);
req.send(null);

If the 3rd argument of the open() method is true, the process is handled synchronously, otherwise synchronously.