A BotNet is a network of machines and devices used to attack other machines and devices.
The owners of the machines in the botnet may have been infected with malware and not aware they are part of it.
As a botnet is aggregated infrastructure, it is also known as ‘malicious cloud’.
In the case of DDoS a botnet is used to send bogus requests to a particular IP address. That is, send protesters to a shop, resulting in no one getting in or out of the street.
It can also be used to send phishing emails at scale. These phishing emails aim to place more malware on more machine. Botnets also hide the identity of the true attacker.