What is a CDN?

A content delivery network (CDN),  refers to a geographically distributed group of servers that work together to provide fast delivery of Internet content.

The goal is to provide high availability and performance by distributing the service spatially relative to end-users.

A CDN allows for the quick transfer of assets needed for loading Internet content including HTML pages, javascript files, stylesheets, images, and videos. The popularity of CDN services continues to grow, and today the majority of web traffic is served through CDNs, including traffic from major sites like Facebook, Netflix, and Amazon.

Source: Wikipedia