[justuk]

Click fraud is the opposite of impression based advertising. Click fraud is when the bad guys encourage people to sign up for services or click on links that offer a sales based bounty and are rewarded for their actions.

Impression based brand advertising is a different model to click based lead advertising. The former places all the risk and work on the adveriser, the later pushes it towards the publisher. If you run a website that's hugely popular and don't want to waste your resources on other business's ad campaigns then selling by impression is the only sensible option.