The purpose of "Denial-of-Service" or "DoS" is to render a service inaccessible.
When a DoS attack is targeted against a website or another service, the effect depends upon the perspective for the attack.
For businesses, it could mean that the online services the businesses depend upon cease to respond. A DoS attack against industrial solutions may compromise control of critical processes or failure of retrieving sensor data. For the average user, a DoS attack will appear as a stop of the service e.g. the website does not respond.
"Distributed-Denial-of-Service" or “DDoS” refers to an attack coming from multiple computers or devices instead of just one.
The purpose of these attacks is to overwhelm the target with a flood of Internet traffic.
Find more information about Denial-of-Service attacks from the pages of National Cyber Security Centre