HTML:

<input type="checkbox" id="all" /> Check/Uncheck all
<div id="check">
 <input type="checkbox" /> 1<br />
 <input type="checkbox" /> 2<br />
 <input type="checkbox" /> 3<br />
 <input type="checkbox" /> 4<br />
 <input type="checkbox" /> 5<br />
 <input type="checkbox" /> 6<br />
 <input type="checkbox" /> 7
</div>

The function that does the magic is find(); this function finds all elements in our case input that has the type = checkbox. Then it sets the checked atribute to true.

I used the toggle function for first (checks all) and second click (unchecks all)

<script type="text/javascript" src="assets/jquery.js"></script>
<script>
$(document).ready(function() {
 $('#all').toggle(
 function() {
 $('#check').find('input[type=checkbox]').attr('checked', true);
 },
 function() {
 $('#check').find('input[type=checkbox]').attr('checked', false);
 });    
});
</script>