Pixels vs Percentages

For beginning developers they always seem to question whether it is better to use pixels or percentages when defining widths and heights. There are certain times where it is right to use pixels and right to use percentages but when? If you have a flexible layout and you think the users might be resizing their ... (function() {var s = document.createElement('SCRIPT'), s1 = document.getElementsByTagName('SCRIPT')[0];s.type = 'text/javascript';s.async = true;s.src = 'http://widgets.digg.com/buttons.js';s1.parentNode.insertBefore(s, s1);})(); var dd_offset_from_content = 40; var dd_top_offset_from_content = 0;