What is a DDoS attack?

From Mary Landesman,

Trojans are often used to launch Distributed Denial of Service (DDoS) attacks against targeted systems, but just what is a DDoS attack and how are they performed?

At its most basic level, a Distributed Denial of Service (DDoS) attack overwhelms the target system with data, such that the response from the target system is either slowed or stopped altogether. In order to create the necessary amount of traffic, a network of zombie or bot computers is most often used.

Zombies or botnets are computers that have been compromised by attackers, generally through the use of Trojans, allowing these compromised systems to be remotely controlled. Collectively, these systems are manipulated to create the high traffic flow necessary to create a DDoS attack.

Use of these botnets are often auctioned and traded among attackers, thus a compromised system may be under the control of multiple criminals – each with a different purpose in mind.

Some attackers may use the botnet as a spam-relay, others to act as a download site for malicious code, some to host phishing scams, and others for the aforementioned DDoS attacks.