Ad fraud is the artificial generation of impressions and clicks (by bots, click farms or fake sites) in order to siphon money from an advertiser’s budget. It inflates costs, understates genuine results and distorts the data you base campaign decisions on. It especially affects programmatic buying, where delivery is spread across thousands of sites that are hard to check by hand. It is curbed by traffic verification, site white and black lists, and brand safety tools. (Compare: Brand Safety, Black List)