Question

if someone just write:

    $("#downloadTabs>div").each(function(el) {
        el.setStyle("display", "none");
        el.removeClass('active');
    });

I would know what is this used for.
But the actually code I read is

    $$("#downloadTabs>div").each(function(el) {
        el.setStyle("display", "none");
        el.removeClass('active');
    });

There is one more $, and what’s this used for ?

Answer 1

The second example is not jQuery, it’s MooTools. The $$ can take a CSS selector to return a set of elements just like the $ function in jQuery:
http://mootools.net/docs/core/Element/Element#dollars

prototype.js also has a very similar $$ function that’s a shortcut to getElementsBySelector:
http://www.prototypejs.org/api/utility/dollar-dollar