If you’re adding some extra jQuery to customize your WordPress site and it doesn’t seem to be working, remember that the copy of jQuery that WP loads is in “no conflict” mode.

If you’re attempting to use the default jQuery shortcut of $, then you’ll want to wrap it with the following:

jQuery(document).ready(function($) {  // $() will work as an alias for jQuery() inside of this function  });

This will save you a lot of time beating your head against a wall, wondering why your jQuery isn’t working. 😉

Source: WordPress Codex