CDNs (Content Delivery Networks) are a global network of very fast servers which can deliver your website very quickly to your visitors, wherever they are in the world. CDNs complement cloud web hosting because they reduce the load on your server and let visitors download files from the most efficient server closest to them.
are a globally distributed network of high performance servers which are used to deliver static content (files, media such as images or video, portions of websites) to users at high speed wherever they are in the world. Wikipedia gives some good general information about CDNs