A distributed denial-of-service (DDoS) attack uses many systems at once to flood a target with traffic or requests until legitimate users can no longer reach it. The attacking traffic may come from botnets, compromised servers, or rented DDoS-for-hire services. Because the load is spread across numerous sources, filtering it can be harder than blocking a single attacker.
DDoS matters because availability is a core security property: even without stealing data, attackers can interrupt business, disrupt services, or pressure victims into paying. In real attacks, DDoS is often paired with extortion, distraction, or political abuse. Defenses usually combine rate limiting, traffic scrubbing, content delivery networks, anycast routing, upstream filtering, and resilient infrastructure so that one flood does not take the service offline.