A DDoS (Distributed Denial of Service) is characterized by the fact that the service receives the attack from different origins, making it more complicated to categorize legitimate traffic belonging to the attack, and then defend from the attack.
Botnets are usually used for DDoS attacks, networks of infected and distributed computers around the world that are used to carry out joint actions. In most cases, the owners of the devices that belong to a botnet are not even aware of it.
For DDoS tests, due to the illegality of controlling a botnet, distributed servers with multiple IPs are used, partially simulating the sum of resources that a botnet would have. The most realistic tests use a large number of different servers and BGP paths, thus maximizing the volume of traffic reaching the target.