In a DoS attack, a hacker attempts to overload or shut down a service so that legitimate users can no longer access it. Typical DoS attacks target web servers and aim to make websites unavailable. No data is stolen or compromised, but the interruption to the service can be costly for a company.
The most common type of DoS attack involves sending more traffic to a computer than it can handle. There are a variety of methods for DoS attacks, but the simplest and most common is to have a botnet flood a web server with requests. This is called a distributed denial-of-service attack (DDoS). (See Backdoor Trojan, Zombie)