A 'botnet' is a system where computer programs are surreptitiously placed onto computers connected to the internet which are later activated by a master computer program. They are used to send spam emails from a large number of unwitting hosts, thus increasing the number of messages sent and making them harder to identify and block for anti-spam software. This site (and associated GitHub library and Botnet Wiki) enables a person to learn about how botnets work. "The botnet built using this laboratory will match the general architecture for any botnet based on a Command-and-Control (C&C) architecture (which) connects to a IRC-Server where all the bots are connected." This is technical, difficult, and engaging stuff. It's worth noting that the author is a 22 year old student.