Clicking ad banners without any intention of purchasing the product. Click fraud is done to make an ad campaign appear more effective. Paying a few cents per hour to workers in a "click farm" or "device farm" in a third-world country to sit at a computer all day and do nothing but click banners makes an ad campaign appear successful. If ads are based on click-throughs (pay-per-click), the website publishing the ads and clicking the ads countless times can make a dishonest profit. In addition, software is available that automatically clicks ads and uses different IP addresses to simulate random users.
In professions such as medical and legal, pay-per-click ads often command a huge premium over banners for common products. A vendor in that field can make its competitor waste a lot of money if it clicks the competitor's ad many times. See pay-per-click