Distributed Denial-of-Service or DDoS is a computerized attack on a website or other online service that causes the service to slow down or shut down, stopping real users from taking advantage of the service.
A DDoS is caused by one computer gaining control of many other computers, unknown to the owners, and directing the computers towards an online service. Because there are many thousands of computers trying to connect with one online service, it becomes overwhelmed and cannot provide services to its real customers.