Distributed systems enable computation and data storage across multiple independent nodes that coordinate to appear as a single coherent system, powering everything from cloud databases and microservices to content delivery networks and blockchain platforms. As systems scale beyond single machines and geographic boundaries, understanding the fundamental tradeoffs—between consistency, availability, and partition tolerance—becomes critical for architects building resilient global infrastructure. This cheat sheet covers the essential concepts that define how distributed systems achieve consensus, maintain consistency, handle failures, and replicate data, providing the theoretical foundations and practical patterns that every engineer working with distributed architectures must grasp to build systems that remain correct under real-world network failures and concurrent operations.