CDN, Content Delivery Network or Content Distribution Network.
Using a CDN will gain you a nice chunk of SEO score with Yslow vastly increases your page loading speed and increased security with a shared SSL. For my website, I am using CloudFlare what offers a single site with free services with an option to buy a subscription on a monthly basis. A CDN will store the content of your website over a cloud network of data centers and then serve the content to the end-user from the closest data center. You will have to create an account with CloudFlare in order to use their services but the first URL will be free if you have more websites add them on another free CDN service. Important to remember that the SSL certificate will take up to 24 hours to be issued to your site do not enable HTTPS links in Joomla and the .htaccess file. If you already own an SSL certificate you can also use that throughout the CloudFlare CDN service make sure to check your settings. The process of adding your website to the CloudFlare service is a click by click process the only real effort is pointing the name server to those of CloudFlare in your domain registrar. Two name servers will be displayed that you need to add in your domain DNS registrar, most likely your host provider CloudFlare has examples of most hosting company's.