Where a Denial-of-Service (DoS) attack is a cyberattack where the perpetrator seeks to make a machine or network resource unavailable to its intended users, a Distributed Denial-of-Service (DDoS) attack is an attack in which multiple compromised computer systems attack a target, such as a server, website, or other network resource. The flood of incoming messages, connection requests, or malformed packets to the target system forces it to slow down or even crash and shut down, thereby denying service to legitimate users and systems.
In a typical DDoS attack, the assailant begins by exploiting a vulnerability in one computer system and making it the DDoS master. The attack master system identifies other vulnerable systems and gains control over them by either infecting the systems with malware or through bypassing the authentication controls (i.e., guessing the default password on a widely used system or device). With a DDoS attack, the incoming traffic flooding the victim originates from many different sources. This effectively makes it impossible to stop the attack simply by blocking a single source.