From Wikipedia:

Click fraud is a type of Internet crime that occurs in pay per click online advertising when a person, automated script or computer program imitates a legitimate user of a web browser clicking on an ad, for the purpose of generating a charge per click without having actual interest in the target of the ad’s link. Click fraud is the subject of some controversy and increasing litigation due to the advertising networks being a key beneficiary of the fraud.

Use of a computer to commit this type of Internet fraud is a felony in many jurisdictions, for example Penal Code 502 in California, USA. There have been arrests relating to click fraud with regard to malicious clicking in order to deplete a competitor’s advertising budget.