Why does JQuery have dollar signs everywhere?

Question

$ is just a shortcut for jQuery. The idea is that everything is done with the one global symbol (since the global namespaces is ridiculously crowded), jQuery, but you can use $ (because it's shorter) if you like:

// These are the same barring your using noConflict (more below)
var divs = $("div");       // Find all divs
var divs = jQuery("div");  // Also find all divs, because
console.log($ === jQuery); // "true"

If you don't want to use the alias, you don't have to. And if you want $ to not be an alias for jQuery, you can use noConflict and the library will restore $ to whatever it was before jQuery took it over. (Useful if you also use Prototype or MooTools.)

$ sign is an alias for jQuery. A short version of jQuery, a less write mechanism.

Just for an example: (in jQuery it's more complicated)

var yourFunction = function() {
    alert('a function');
}

window.Myf = yourFunction;

Now you can call yourFunction like:

Myf(); // definitely a short syntax

It's just a convenient character, shorter to type and easier to read than "jQuery".

There is nothing special except that it's traditionally not used to start a variable or function name, which reduces the risk or name collision.

Writability and Performance

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When we are working on library or a programming language we should pay attention to some writability rules. Thanks to jQuery they already implemented lots of options. You can use $ or you can use jQuery or you can use _

(function (_) {
    _("#wow").click()
})(jQuery);

Or maybe you can do fancy changes, javascript identifiers are unicode so you can use Ω

(function (Ω) {
    Ω("#wow").click()
})(jQuery);

But the main idea behind it, pressing once to the keyboard is better than typing jQuery

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On the other side, we have performance... I just randomly opened one of my projects and searched for $, I used 54 $ in a single javascript file.

$ is a byte.

jQuery is 6 bytes.

The difference is huge 54 * 5 = 220 bytes.