The 'id' is used when the style to be applied is unique, .i.e., occurs once in the whole of the program code.

Code:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
 <HEAD>
  <TITLE> Learning CSS : id</TITLE>
  <link rel="stylesheet" href="css/style.css" 
type="text/css" media="screen,projection">
 </HEAD>

 <BODY>
  
<div id="footer">
    Copyright  &copy;  2009 BizHat.com. All Rights Reserved<BR>
    Designed and Hosted by 
    <a href="http://www.flashwebhost.com" target="blank">
    FlashWebHost.com</A>
</div>

 </BODY>
</HTML>
The above code occurs only once in the site code, So we use 'id' named 'footer'. The style will be defined in a file named style.css and stored in css directory at the root. Here 'a' defines the link text colour.

css/style.css

Code:
a
{
    color:#FFFFFF;
}

#footer 
{
    font-family: verdana;
    font-size:10pt;
    width:850px;
    margin:0 0 0 0;
    background:#3366CC;
    color:#FFFFFF;
    text-align:center;
    padding-top:10px;
    padding-bottom:10px;
}