A distributed denial-of-service (DDoS) attack involves a large number of computers that have been hijacked and networked into a botnet.
This collection of compromised systems then attack a single website, grinding it to a halt, and thereby denying service to all those internet surfers who wish to visit said website.
In essence, a DDoS attack will flood the target system’s server with information and requests which will eventually cause it to shut down and, therefore, deny the normal service to legitimate users.
The person, or syndicate, behind a DDoS attack will use a botnet, comprising of one vulnerable computer that will act as the DDoS master and, potentially, millions of other hijacked machines which will act as the master’s slaves.
The controller of the botnet can then issue a single command through the master, directing all the slaved machines in the network to flood the target system many times over and for as long as they feel is necessary to bring the object of their attention to its knees.
This perpetual bombardment of packets of data will eventually lead to a complete denial of service.