A Content Delivery Network (CDN) is a distribution network of different servers located worldwide to deliver data fast and efficiently over the Internet.
Streaming as we know it would not work without a CDN. To stream any content, you need your origin file stored at a server. The viewer’s device sends a request to the source, and the origin server delivers the data to the client. But in the case of video-streaming, you often need a distribution network to send the data fast and reliable.
Compared with most files you can request from a server, e.g., a specific website, an email, or even a photo library, video data is much bigger. So it needs an excellent digital infrastructure to find its way through the Internet. If data is sent to a large number of viewers (a film on Netflix) or the data volume is enormous (a 4HD video file), a single server cannot meet the requests. If too many users access a single source server simultaneously, it is quickly overloaded. But even without overload, longer buffering times for data transfer can occur if the origin server is placed far away from the client.
A content delivery network is a solution to this problem: It consists of many different distribution servers that can spread the requested data worldwide and cache it. For example, if you want to stream a sports game in New York, the original server is probably located very close. To provide thousands of viewers all over the US with a high-quality stream without long buffering times, a CDN would cache the stream data on some fewer servers (maybe one in each state). From then on, the file is distributed again to other servers in the state. In the end, high video quality and less buffering times are ensured by distributing the client requests to many different servers, ideally as close as possible to the viewer.
Some of the world’s most popular CDNs are Akamai Technologies, Amazon CloudFront, CDNetworks, CenturyLink, Cloudflare, and many more.