which is better? CDNs or copy on server? [on hold]

I am new to web development I wanted to know which is better for production websites linking frameworks(jquery,bootstrap etc..)files via CDN's(Content delivery Networks) or linking the files within the server?

Eg: linking Jquery

<script type="text/javascript" src="http://ift.tt/2a1eCsp"></script>

Or

<script type="text/javascript" src="js/jquery.min.js"></script>