ajax.js

jQuery(function ($) {
    var $mainContent = $('#main-content'),
        $cat_links = $('.category-filters li a')
    ;

    $cat_links.on('click', function (e) {
        e.preventDefault();
        var $el = $(this),
            value = $el.attr("href")
        ;

        $mainContent.animate({opacity: "0.5"});
        $mainContent.load(value + " #main-content", function () {
            $mainContent.animate({opacity: "1"});
        });
    });
});