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.
Some web hosting services does have a black list of Internet protocol addresses of some countries, all of the visitors with a blocked IP address by those kinds of services could not connect to remote server of the websites that hosted by the web hosts where they're blocked. They were unable to reach that site if their IP is blocked. What if the visitor who have linked to your site has a blocked IP address on your web host? That visitor will not going to reach your site but will be warn by their web browser leaving a message such like “You tried to access the address which is currently unavailable.” As far as I know, by using content delivery network can also increase the global availability of your site because they'll replicate your content with their servers in different countries. If your server has blocked the IP addresses of your visitors, some kind of CDNs have a feature that will serve them a limited copy of your cached web pages to keep it online for those visitors, but not all distribution networks have the same features.
For you to know much better how the content delivery network serves the content of their user to the visitors, try to open or view one of the images of the CDN using website that you knew. Instead you've linked to the image file like http://www.heart.tk/images/sample.jpg address, you will be linked to http://cdn.heart.tk/images/sample.jpg a subdomain which hosted by the content delivery service provider. (Note: Those files' addresses are not exist, they're just an example!) But it doesn't mean that all the sites that using a CDN have the cdn. subdomain of their domain name, it is in their content delivery provider if they can customize it. Some other networks delivers a content with the domain name that they've own.