Difference between jquery and $

when .noConflict() is called, selector like $('') is no longer working to ensure compatibility with other framework such as Prototype. at that time jQuery('') is used instead.

Reference: jQuery.noConflict()

To better illustrate the idea, here is an example obtained from the reference link:

<script type="text/javascript">
  $.noConflict();
  jQuery(document).ready(function($) {
    // Code that uses jQuery's $ can follow here.
  });
  // Code that uses other library's $ can follow here.
</script>

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

You can get detailed information related to it from its Documentation