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A large distributed system of servers that exist in different geographic locations with copies of data, once user request a data he will get it from the nearest server to him instead of having this data from the main server which help in serving with high availability and high performance and reduce the delay.
Wikipedia - Content delivery network
It makes sense to use a CDN for largely static content - namely your server would be relieved of having to load the jquery files on every page request. All static consistant content like images, sounds files, javascript libraries and css libraries etc is best hosted via a CDN - this would basically help speed up your site.
CDN :
It is a network of distributed system of server located all over the internet.
content serve to end-users with maximum availability and high performance. It provide large fraction of the Internet content like web objects, downloadable media files, software, e-com applications live streaming on-demand video, and social networks.
The benefit of loading JS library from CDN is that it make sure that you always get the latest version of JS lib file also user don't need to load JS file each time they request becz most of the time it will already in users browser cache that have already loaded from CDN for other website.
A CDN is form of cache memory basically. Your operating systems caches the recently used data from the hard disk. This is because accessing the hard disk each time reduces speed.
Similarly, a CDN caches most used resources of a webpage. This can be jquery files, css files or even image or video files. In this way, if your server is in a remote place; your page load is not hampered because users load your page from the local CDN.
The CDN company normally have several server sources around the world. Your webdata is cached and copies are spread aorund the world. Any user who loads your website gets its from the nearest point.
So, it's a great idea to have CDN implemented in your website. Google's SEO update says that faster websites rank higher in web searches.