Example:1 Source Code Explained
Source Code Explained:
To get the Element of a ID ----> document.getElementById("hello");
the above code uses to get the element id with hello.

Step : 1
<div id="hello"></div>

Step : 2
The document.getElementById("hello"); 
Used to get the id name of the particular div. 

Step : 3
var x =document.getElementById("hello");
Storing the retrieved document id to variable 'x'

Step 4: 
innerHTML method in java script used to display a Dynamic Content.

For Example:
Will result in display a Bold word as Kannan.

Step 5:
Checking for Old Browser and New Browser

xmlhttp = new XMLHttpRequest();
xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");

Step 6:
// code

There are five Ready states in Ajax as i mentioned before. 
onreadystatechange is the method which will executed if we get successful request from server.

Step 7:
 The above coded used to open a connection.
It must be given in all ajax codes. Else it won't Work.
Note :
Inside xmlhttp.open i gave POST method instead of that you can use GET. 
And you have to the link of your own site which you going to put code. 
I mentioned http://ajaxtutorialonline.blogspot.com because i coded the program for this site. If you put your code in any website you can put your own website or domain name ex : http://example.com

Step 8 :
The send method will send the data. If you not retrieve any data you can simply give null.