A DoS attack (Denial of Service attack) is a malicious cyberattack intended to overwhelm a system with requests, making it unavailable for legitimate users. It is used to disrupt services and cause denial of access to resources, like websites or apps. It exploits vulnerabilities in networks or software to flood the system with requests or data packets, using up its resources and making it unresponsive. Companies must be aware and have strategies in place to stop such attacks as they can lead to significant losses in revenue and reputation.