Click fraud consists of “pay-per-click” ads or in other words, repeat clicking of such ads. It has a deep impact on any advertisements, mainly paid ads. There are two categories of such abusive clickers: one who does not affect your ads directly but uses the paid ads without letting you know about its whereabouts.
The second one is the most malicious one. This is the one which has to be avoided. These are “serial” abusers.