All the features I've detailed to this point are valuable to the web hosting experience, but none matches the critical importance of site uptime. If your site is down, clients or customers will be unable to find you or access your products or services. It doesn't matter how great the features are, or how good it looks; if your site is down, it might as well not exist.
A Content Distribution Network (CDN) is a network of servers distributed geographically. The purpose of a CDN is to deliver content on the internet much faster. If a user in Toronto is trying to access your website, the closest server to them will take care of this. But someone coming to your website from Sydney, Australia, will be catered to by a server closest to them. The main host server holds your website content, and all the other servers have a cached version each.