Content delivery networks (CDNs) serve static assets via globally distributed servers to improve web app loading speed.
Mastering HTTP Caching is a fantastic post that goes into great technical detail on how CDNs and caching work.
MaxCDN vs CloudFlare vs Amazon CloudFront vs Akamai Edge vs Fastly compares and contrasts the most popular CDN services based on features, performance and pricing. Note that Full Stack Python uses Cloudflare to serve all content.
Building your own CDN for Fun and Profit is a great high-level overview of how CDNs work and shows you how to create your own, albeit simplified CDN.
Do not let your CDN betray you: Use Subresource Integrity describes the security implications for CDNs with unexpectedly modified content and how Subresource Integrity in modern web browsers can mitigate this vulnerability if used properly.