Denial-of-Service attack

DoS attacks consist of surcharging a system to make it slower, and eventually unavailable. It's typically accomplish by flooding the system, where a lot of fake superfluous requests prevent the legitimate ones to be fulfilled.

You can think of it as a group of people crowding the entry door of a shop, making it hard for legitimate customers to enter, thus disrupting trade and losing the business money.

In Distributed Denial-of-Service (DDoS) attacks, the flooding come from many different sources (i.e. computers, servers, …).