Drupal 7: Proper implementation of jQuery in a Drupal theme.

d7_theme_jquery_usage.js

(function ($) {
  Drupal.behaviors.themename = {
    attach: function(context, settings) {
    // Add a hover class to nav items.
    $('.mainnav nav a')
      .hover(
        function() {
          $(this).addClass('hover');
        },
        function() {
          $(this).removeClass('hover');
        }
      );// end hover
  }}
}) (jQuery);