This document provides an overview of key concepts in distributed systems including: 1) The CAP theorem which states that a distributed system cannot simultaneously provide consistency, availability, and partition tolerance. 2) Consistency models such as strong, weak, and eventual consistency. Eventual consistency guarantees that if no new writes are made, reads will return the last updated value. 3) Consensus protocols like Paxos which allow distributed systems to agree on a value despite potential failures. Vector clocks are used to order events in a distributed system.