What is DDoS (Distributed Denial-of-Service)?
DDoS is an advanced DoS attack intended to deny legitimate users access to web services by overwhelming the relevant web server(s) with requests at a scale far higher than legitimate web service use will warrant.
Whereby a DoS mitigation system might throttle rapid and malicious connection attempts from a specific client/IP, a DDoS attack employs multiple clients/IPs in flooding a server with requests. This makes defending against this kind of attack very difficult without taking server(s) completely offline.
Advanced measures, including connection and connection rate limiting and dynamic IP blacklisting, can be employed to minimize the effectiveness of DDoS attacks.