There has been several stories circulating around the internet about botnets and how they can take down a companies website. I read a great post from welivesecurity.com explaining what a botnet is and how it works. Here is a quick definition from the post:
The word botnet is made up of two words: bot and net. Bot is short for robot, a name we sometimes give to a computer that is infected by malicious software. Net comes from network, a group of systems that are linked together. People who write and operate malware cannot manually log onto every computer they have infected, instead they use botnets to manage a large number of infected systems, and do it automatically. A botnet is a network of infected computers, where the network is used by the malware to spread.
If you would like more information you can read the entire post here.