Caching is a fundamental performance optimization technique in backend systems that stores frequently accessed data in temporary, fast-access storage layers closer to the application. By reducing redundant database queries and expensive computations, caching can cut response times from seconds to milliseconds and reduce database load by 90% or more. The critical challenge lies not just in storing data, but in choosing the right caching pattern, managing invalidation, and preventing common failure modes like cache stampedes and stale data — all while maintaining consistency across distributed systems.