The jQuery Form Plugin allows you to easily and unobtrusively upgrade HTML forms to use AJAX. The main methods, ajaxForm and ajaxSubmit, gather information from the form element to determine how to manage the submit process. Both of these methods support numerous options which allows you to have full control over how the data is submitted. Submitting a form with AJAX doesn’t get any easier than this

Form Plugin API
The Form Plugin API provides several methods that allow you to easily manage form data and form submission.

ajaxForm
Prepares a form to be submitted via AJAX by adding all of the necessary event listeners. It does not submit the form. Use ajaxForm in your document’s ready function to prepare your form(s) for AJAX submission. ajaxForm takes zero or one argument. The single argument can be either a callback function or an Options Object.
Chainable: Yes.

how tu use


<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
 <HEAD>
  <TITLE>Jquery post </TITLE>
  <META NAME="Generator" CONTENT="EditPlus">
  <META NAME="Author" CONTENT="">
  <META NAME="Keywords" CONTENT="">
  <META NAME="Description" CONTENT="">

<script type='text/javascript' src="jquery-1.2.6.js"></script>
<script type='text/javascript' src="jquery.form.js"></script>
  <SCRIPT LANGUAGE="JavaScript">
  <!--

	$(document).ready(function() {
    $('#htmlForm').ajaxForm({
		beforeSubmit: validate,
        target: '#htmlExampleTarget', 

        success: function() {
            $('#htmlExampleTarget').fadeIn(1000);
			$('#htmlExampleTarget').fadeOut(3000);
        }
    });
});

function validate(formData, jqForm, options) { 

    var name = $('input[@name=message]').fieldValue();
    var check = $('input[@name=xxx]').fieldValue(); 

    if (!name[0] || !check[0]) {
        alert('Please enter a value for both');
        return false;
    }
}

  </SCRIPT>
<style type="text/css">
.style {
	font-size: 10px;
	font-family: Verdana, Arial, Helvetica, sans-serif;
	color: #000000;
}
</style>
</HEAD>

 <BODY>
  <form action="echo.php" method="post" class="style" id="htmlForm">
    Message: <input type="text" name="message" value="Hello HTML" />
    <INPUT TYPE="checkbox" NAME="xxx" value="xxxxx">
    check this
    <input type="submit" value="Echo as HTML" />
 </form>
<div id="htmlExampleTarget" class="style"></div>
 </BODY>
</HTML>

echo.php


<?php
echo '<div style="background-color:#ffa; padding:20px">' . $_POST['message'] . ' '.$_POST['xxx'].'</div>';
?>

More info
http://www.malsup.com/jquery/form/