jQuery is the starting point for writing any jQuery code. It can be used as a function jQuery(...) or a variable jQuery.foo. $ is an alias for jQuery and the two can usually be interchanged for each other (except where
jQuery.noConflict(); has been used - see Avoiding namespace collisions).
Assuming we have this snippet of HTML -
<div id="demo_div" class="demo"></div>
Now we might want to add some text on the div content using jQuery. To do that we can either use jQuery function or $ -
jQuery('#demo_div').text('Demo text');
or
$('#demo_div').text('Demo text');
Both will result the same final HTML -
<div id="demo_div" class="demo">Demo text</div>
As $ is more concise than jQuery it is generally the preferred method of writing jQuery.