A geographically distributed network of servers that caches your site's files and serves them from the point closest to each visitor.
CDN stands for content delivery network. Instead of every customer reaching one distant server, copies of images, style files and static pages are cached on servers in many cities, and each visitor gets them from the nearest one. The result is faster loading and less load on your main server.
Speed affects customer experience and sales, especially in image-heavy stores. CDNs also help absorb traffic spikes and block some kinds of attacks. Latency for dynamic data still depends on where your server and database are located. Practical tip: compressing and resizing images delivers big gains even before a CDN. Make sure user-specific pages, like the account page or cart, are not cached on the network, or one customer's data could appear to another.