DDoS Distributed Denial of Service
Many computers simultaneously flood a server with requests so it dies under the load.
"Distributed" = from thousands of infected devices (botnets) instead of one attacker. Different from pure DoS, which comes from a single source.
Protection: CDNs like Cloudflare absorb a lot of the traffic, per-IP rate limiting, anomaly detection. Large attacks require dedicated DDoS protection services.