If a website is big and have a lot of large files, such as high quality JPEG image, MP3 audio, and Mp4 video files, that kind of site needs CDN. It is an abbreviation for the word, content delivery network also known as content distribution network. It's an online service that has servers around the world, who takes the static content of their client's site, such like multimedia, cascading style sheet (CSS), and JavaScript files, downloadable objects and applications, or even the dynamic content and replicate them through their servers around the globe instead of a single server of their client's web hosting provider at one location. Webmasters and bloggers use this technology to reduce the bandwidth costs of their sites and weblogs, to improve the loading speed of their web pages and blog posts, and to increase the global availability of their content.
If you are a webmaster, reducing the bandwidth costs of your website or weblog is one of the most important things you should knew if your web hosting or blog software plan has a bandwidth limit. Because if you reach your bandwidth limit, all of the web pages and/or blog posts of your site or blog will not work leaving a message like “The server is temporarily unable to service your request due to the site owner reaching his/her bandwidth limit. Please try again later.” A content delivery service can reduce your bandwidth costs by replicating the static content (or even the dynamic content) of your site into their server, and attach the replicated content to your site. Therefore, the bandwidth costs for loading the multimedia files or any downloadable objects on your site will minimized because your content delivery (or CDN's) server will do the work to deliver your static files on your visitor's web browser. Some content distribution networks block threats and limit abusive bots and crawlers from wasting your bandwidth resources. Normally a CDN has also a bandwidth limit, but the advantage is, you've doubled your traffic limit and stopped an abuser from wasting your bandwidth.
If the location of your web hosting provider is closer to your location, the loading speed of your website when you view it, is probably faster than to the viewer who is in the other country. It is because your web hosting server was near to your place. What if your web host is far from you and your visitors? A content delivery network replicate the static content or even the dynamic of your site makes the time speed of your web page fast, because basically a CDN has servers around the world. So, if your content delivery provider has a server near to the city of your viewer, your page that the viewer have visit will load faster. Each visitor to your site gets their data from your content delivery server that closest to them. So, if you want to increase the loading speed of your site, then use CDN, it is one of the techniques on search engine optimization, because search engine such Google loves fast sites.