Botnet is a network of infected devices used by hackers and attackers to organize DDoS attacks and mass mailings. In some cases, the computing power of infected devices is used for cryptocurrency mining.
Most often, the infected network is managed by a special server. In some cases, a botnet may have a decentralized structure organized according to the P2P principle. An infected network may consist of a huge number of devices. In some cases, the number can reach several millions.
The owner of an infected device may not even realize that his property is being used by attackers. Most often, botnet software is a combination of a Trojan and a rootkit. It can remain in the system for a long time without manifesting itself in any way, exactly until an activation command is received from the outside. Sometimes the infection can be detected by indirect signs: increased load on computing resources and increased Internet traffic.
The ways of infection are the same as in cases of regular computer viruses: e-mail, pirated software, infected websites, clicking on dubious links. Antivirus programs are a relatively effective way of protection, but do not provide a full guarantee of security.