What is the fundamental difference between how a Denial of Service (DoS) and a Distributed Denial of Service (DDoS) attack operate?

A DoS attack originates from a single source, overwhelming a target system with traffic. A DDoS attack, however, leverages multiple compromised computer systems (often a botnet) to simultaneously flood the target, making it far more difficult to mitigate. Essentially, DDoS amplifies the impact of a DoS attack through distributed participation.