Formation

This example illustrates how a botnet is created and used to send email spam.

How a botnet works
A botnet operator sends out viruses or worms, infecting ordinary users' computers, whose payload is a malicious application—the bot.
The bot on the infected PC logs into a particular C&C server.
A spammer purchases the services of the botnet from the operator.
The spammer provides the spam messages to the operator, who instructs the compromised machines via the control panel on the web server, causing them to send out spam messages.

Botnets are exploited for various purposes, including denial-of-service attacks, creation or misuse of SMTP mail relays for spam (see Spambot), click fraud, mining bitcoins, spamdexing, and the theft of application serial numbers, login IDs, and financial information such as credit card numbers.

The botnet controller community features a constant and continuous struggle over who has the most bots, the highest overall bandwidth, and the most "high-quality" infected machines, like university, corporate, and even government machines.[8]