What is the difference between $ and jQuery

Question

When I try to use $("#div_id") in $(document).ready it returns NULL, but when I use jQuery("#div_id") it returns the actual object!

Why is that happening?

UPDATE: I tried noConflict method without gaining any hints.

jQuery.noConflict()
function (a,b){return new c.fn.init(a,b)}

$.noConflict();
TypeError: Object function ()
    {
        return document.getElementById.apply(document, arguments)
    } has no method 'noConflict'

UPDATE 2:

$(document).ready(function() {
    debugger;
});

<input type="text" id="test" name="test" value="123" />

When I run the following code in the console i got those results:

$("#test").val()
TypeError: Cannot call method 'val' of null
jQuery("#test").val()
"123"

Thanks

Answer 1

See jQuery.noConflict(). Could other javascript libraries on your page be using the $ variable?

$ is just a variable that is used to alias jQuery and being a variable, anything could be assigned to it.