A Denial of Service (DoS) attack is a malicious attempt to disrupt the normal functioning of a targeted server, service, or network by overwhelming it with a flood of traffic. This excessive traffic can prevent legitimate users from accessing the service, effectively making it unavailable. In a more advanced form, known as a Distributed Denial of Service (DDoS) attack, multiple compromised computers are used to launch the attack simultaneously. This makes it harder to defend against, as the traffic comes from many different sources, complicating the task of identifying and blocking the malicious activity.