A Content Delivery Network (CDN) caches static content at edge servers worldwide.
Instead of every user fetching images from your server in Virginia, users in Tokyo fetch from a CDN server in Tokyo. Latency drops from ms to ms.
Use CDNs for: images, videos, CSS, JavaScript, fonts.
Don't use CDNs for: personalized content, real-time data, API responses (usually).
Popular CDNs: CloudFlare, AWS CloudFront, Akamai, Fastly.
In interviews, mention CDN whenever the system serves static content to global users.