DDoS attack consists in "overloading" the attacked server or communication channels, and the exhaustion of the resources of the attacked system either degrades the quality of the application or stops its operation altogether. Simple DoS is rare and is usually fought off by banal blocking of access to the system for the attacking computer.
DDOS is a distributed attack based on the same principle; it is carried out from more than one attacking computer

More info @ https://www.extnoc.com/blog/what-is-a-managed-ddos

ddos as a service