DoS vs DDoS Attacks.

A Denial of Service or DoS attack is when someone crafts a request, tailored to programs on the target service, with the aim of bringing it down. This makes it similar to a real hack.

The request originates from a single machine.

A DDoS attack on the other hand is a large number of often identical requests, from hundreds of different machines with the aim of flooding a service so legitimate users cannot access it.