What is a DDoS attack and how is it mitigated?

DDoS (Distributed Denial of Service) overwhelms target with traffic from many sources. Types: Volumetric (flood bandwidth - UDP flood, amplification), Protocol (exploit protocol weaknesses - SYN flood), Application layer (target services - HTTP flood). Mitigation: CDN/edge filtering (absorb traffic), rate limiting, IP reputation/blacklisting, CAPTCHA for bots, anycast distribution, specialized DDoS protection (Cloudflare, AWS Shield), overprovision capacity. Defense requires multiple layers.