Denial-of-Service Attack

A Denial-of-Service Attack, commonly abbreviated at DoS or DDoS, is a cyber attack in which the attacker attempts to bring down an organization's services by either overloaded their servers with bogus requests or clogging the user's bandwidth.

DDoS stands for distributed denial of service which means that an attacker is using multiple devices to launch the attack as opposed to one source. This can be done through a botnet or other forms of zombie computer techniques.

References:
https://www.us-cert.gov/ncas/tips/ST04-015