What is Content Delivery Network (CDN)?

Question

What is Content Delivery Network and what does it do?

Answer

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

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.

A properly configured CDN may also help protect websites against some common malicious attacks, such as Distributed Denial of Service (DDOS) attacks.

Source: What is a CDN? -Cloudflare

There are quite a few popular CDN: 

Cloudflare, Sucrui, KeyCDN