Botnet traffic is an artificial traffic generated from thousand of infected zombies PCs - some botnet may count more than one million PCs - and aiming, among other things, at generating fraudulent advertising revenue through click fraud and impression fraud.
Zombie PCs are a "mafia practice" by which PCs are hacked with trojan horse programs. Visits and clicks are made on the PC without the owner being aware. Even if a user was sitting at the computer, he would not see the hidden browser.
In February 2013, Spider.io discovered a botnet called Chameleon which emulated human visitors on 202 selected websites. According to Spider.io estimates:
Chameleon counted 120 000 of host machines
the botnet caused 9 billions of display ad impressions to be served per month to infected PCs
the botnet generated an average click-through rate of 0.02%
the monthly cost to advertisers of ad impressions served to the botnet was at least $6.2 million
The botnet signature:
Image credit Spider.io
Other figures for ZeroAccess botnet analyzed by Sophos
Sophos Technical Paper: ZeroAccess Botnet