<style>
  input.csson { color: red; }
  input.cssoff { color: gray; }
</style>
<form>
  <input id='author' type='text' value='' rel="writehere|type author|cssoff|csson"/><br/>
  <input id='topic' type='text' value='' rel="writehere|type topic|cssoff|csson"/><br/>
</form>


function setWriteHere(search) {
	/* add on click event handler */
	$("input[rel^="+search+"]").click( function () {
		var ar = $(this).attr('rel').split('|');
		if($(this).val()==ar[1]) { $(this).val(""); $(this).attr('class',ar[3]); }
	} );
	/* add on blur event handler */
	$("input[rel^="+search+"]").blur( function () {
		var ar = $(this).attr('rel').split('|');
		if($(this).val()=="") { $(this).val(ar[1]); $(this).attr('class',ar[2]); }
	} );
	/* trigger blur event */
	$("input[rel^="+search+"]").each( function () { $(this).blur(); } );

}

$(document).ready(function() {
	/* fix forms when ready */
	setWriteHere("writehere");
	}
);

http://www.barattalo.it/2009/11/17/set-write-here-on-input/

When you click on the text box the default value (such as "write here") disappears and you can type in. On blur if the textbox is empty it will appear the default value again.

"Insert your email here" with javascript and JQuery

jquery

jQuery