Who Commits Click Fraud?
Sometimes click fraud is perpetrated by well-meaning persons, such as friends and family. This is why programs like AdSense have rules forbidding web sites from drawing undue attention to the ads. Some sites outright ask visitors to click ads as a show of support, but such sites risk being banned from their ad program for failing to comply with the program's rules.
Large-scale click fraud is a bigger problem. Defrauders (often referred to as fraudsters) literally hire people, often in a foreign country with cheap labor, to click ads. Or they use networks of zombie computers (computers controlled remotely by unauthorized parties due to virus or spyware infection) to click the ads.