Caching 101
A cache is a copy of data kept somewhere faster or cheaper to access than the place that data actually lives, so that a repeated request can be answered from the copy instead of redoing the original expensive work. It's arguably the single highest-leverage tool in system design, precisely because it doesn't require changing the underlying system at all — a cache sits in front of something slow and simply intercepts requests before they get there.
Caching Strategies
Caching 101 covered the basic hit/miss mechanism; a caching strategy is the answer to the two questions that mechanism leaves open — who's responsible for populating the cache, and when does a write update it? Different strategies answer those questions differently, and the choice has real consequences for staleness, latency, and what happens if the cache goes down.
Cache Eviction Policies
A cache is almost always smaller than the data it could possibly hold — that's often why it's fast, since keeping everything in a small, high-speed memory tier is what makes lookups quick in the first place. Once it's full, adding a new entry means something else has to leave. An eviction policy is the rule that decides what to remove, and which rule you pick has a direct, measurable effect on hit rate.
Distributed Caching
A single cache server has the same ceiling any single machine does: finite memory, finite throughput, and it's a Single Point of Failure — if it goes down, every request behind it falls straight through to the database at once. Distributed caching spreads the cache itself across multiple machines, applying the same horizontal scaling idea already covered for application servers and databases to the caching layer.
CDN
A CDN (Content Delivery Network) is caching applied to geography: a network of servers, called edge servers or points of presence (PoPs), physically distributed around the world, each holding a cached copy of content close to the users who request it. Every other cache in this module has been about skipping work (a database query, a computation); a CDN is specifically about skipping distance — the physical time it takes a signal to travel.