What is Botnet?
A botnet or robot network is a group of computers running a computer application controlled and manipulated only by the owner or the software source. The botnet may refer to a legitimate network of several computers that share program processing amongst them.
Various types of Botnets any day can infect your computer which will become one in the army of botnet. mostly botnets are used for denial or service attacks, spamming and monittoring traffic, keyloging and mass identify theft or simply spreading the botnet itself.
You can read more about it here.
Here is an article of the newest take down of monster botnet which held 800,000 computers.