CDN is short form for Content Delivery Network. Content Delivery Network is a network of servers that delivers web pages to the users according to their geographical location. A CDN copies the pages of a Web site to a network of servers that are dispersed at geographically different locations, caching the contents of the page. When a user requests a Web page that is part of a CDN, the CDN will redirect the request from the originating site's server to a server in the CDN that is closest to the user and deliver the cached content. The CDN will also communicate with the originating server to deliver any content that has not been previously cached.
This service is effective in speeding the delivery of content of Web sites with high traffic and Web sites that have global reach. The closer the CDN server is to the user geographically, the faster the content will be delivered to the user. CDNs also provide protection from large surges in traffic
The process of bouncing through a CDN is nearly transparent to the user. The only way a user would know if a CDN has been accessed is if the delivered URL is different than the URL that has been requested.