Denial Of Service Attack:
An assault on a network that floods it with so many requests that regular traffic is either slowed or completely interrupted. Unlike a virus or worm, which can cause severe damage to databases, a denial of service attack interrupts network service for a long period of time.
A distributed Denial of Service attack can employ hundreds or even thousands of computers that have previously been infected. The computers act as “zombies” and work together to send out bogus messages, thereby creating huge volumes of phony traffic.