Content Delivery Networks (CDNs) are globally distributed networks of edge servers that cache and deliver web content from locations nearest to end users, dramatically reducing latency and origin server load. CDNs sit between users and origin servers, serving as intelligent reverse proxies that make routing decisions based on real-time performance metrics, geographic proximity, and server health. The fundamental trade-off in CDN architecture is between cache hit ratio (how often content is served from cache) and content freshness — this tension shapes every caching policy, TTL setting, and invalidation strategy you'll configure.