Using JQuery in Drupal 7

From the Drupal 7 upgrade guide:

Javascript should be made compatible
with other libraries than jQuery by
adding a small wrapper around your
existing code:

(function ($) {
  // Original JavaScript code.
})(jQuery);

The $ global will no longer refer to
the jquery object. However, with this
construction, the local variable $
will refer to jquery, allowing your
code to access jQuery through $
anyway, while the code will not
conflict with other libraries that use
the $ global.

You can also just use the ‘jQuery’ variable instead of the $ variable in your code.