Caching is a fundamental performance optimization technique that stores copies of frequently accessed data in fast-access storage layers, reducing latency from milliseconds to microseconds and dramatically decreasing load on backend systems. Effective caching sits at the intersection of data locality, consistency models, and eviction policies—choosing the wrong strategy can create data staleness issues or cache stampedes that bring down entire systems. The key insight: caching is not just about storing data faster; it's about intelligently deciding what to cache, when to invalidate it, and how to handle failures when the cache becomes stale or unavailable.