Well, the difference between a DoS and a DDoS attack is generally that a DoS attack is performed from a single computer. It can still bring down relatively large sites, especially if they already have a heavy user load. A DDoS attack can bring down just about any site if there are enough users to perform the attack. One user can pretty much log onto the target site with an average of a thousand simulated connections. So, imagine what that does if a DDoS attack with a thousand of computers involved target a server. Not many servers can handle a million users.

The DDoS technique was used in the MyDoom virus. Generally, all users infected were set to attack huge sites, such as microsoft.com and bring them down.

"Stephen Wolfram is the creator of Mathematica and is widely regarded as the most important innovator in scientific and technical computing today." - Stephen Wolfram