A denial-of-service attack overwhelms a system’s resources so that it cannot respond to service requests. A DDoS attack is also an attack on system’s resources, but it is launched from a large number of other host machines that are infected by malicious software controlled by the attacker.
Unlike attacks that are designed to enable the attacker to gain or increase access, denial-of-service doesn’t provide direct benefits for attackers. For some of them, it’s enough to have the satisfaction of service denial. However, if the attacked resource belongs to a business competitor, then the benefit to the attacker may be real enough. Another purpose of a DoS attack can be to take a system offline so that a different kind of attack can be launched. One common example is session hijacking, which I’ll describe later.
There are different types of DoS and DDoS attacks; the most common are TCP SYN flood attack, teardrop attack, smurf attack, ping-of-death attack and botnets.