A distributed denial of service (DDoS) attack uses multiple infected devices to target and overwhelm the resources of a single network, application or service, denying access to legitimate users.
Infected equipment could include personal or business computers, servers or even Internet of Things (IoT) devices - which was the case in the DDoS attack on Dyn.
Hackers use those Trojans to turn the devices into an army of botnets. When triggered, they act as zombies to overwhelm a target and cut off legitimate Internet traffic.
Take a look at the video below to see how this type of cyberattack works.