Quick Definition
A Distributed Denial of Service (DDoS) attack is an attack designed to stop an online system's normal operation by creating a "traffic jam" of sorts.
DDoS attacks are generally carried out by directing a large number of network-connected computer systems, usually controlled by a botnet, to make network requests of the target all at the same time. This has the effect of overwhelming its bandwidth or processing resources, rendering it unable to carry out normal work for the duration of the attack and often some time afterward.
