Question

Hey guys, what function could I use to say, if this radio button is checked show this ‘div’.
Thanks in advance.

Answer 1

HTML

<form id='form-id'>
    <input id='watch-me' name='test' type='radio' /> Show Div<br />
    <input name='test' type='radio' /><br />
    <input name='test' type='radio' />
</form>
<div id='show-me' style='display:none'>Hello</div>

jQuery

$('#form-id').change(function() {
    if ($('#watch-me').attr('checked')) {
        $('#show-me').show();
    } else {
        $('#show-me').hide();
    }
});

You can see it in action here: http://jsfiddle.net/wmKGd/

UPDATE 25.01.2013
After Upgrading from jQuery Library 1.8.x to 1.9.x
Please use instead of
jQuery Library 1.8.x

jQuery('#some-id').attr('checked') 

jQuery Library 1.9.x

jQuery('#some-id').prop('checked')

You can see it with updated script in action here: http://jsfiddle.net/9XXRY/