Pay per click is an advertising model used in affiliate programs, meaning that the affiliates will get paid when a user clicks on an ad. In pay per click affiliate programs, the merchants will pay the affiliate for every click the affiliate manages to provide.

Indeed, this type of model is not so popular as it used to, not because affiliates don't find it attractive enough, but because merchants have been a subject to click fraud incidents, while having almost no click fraud detection mechanism, they had find more appropriate models to pay their affiliates- such as ones that pey for a complete action and not just a click or impression. 

But luckily, some advertisers still use this method in their affiliate program. These advertisers have concluded that they end up paying less per click to the affiliate, than paying to the search engines, such as Google Adwords. In other words, they are willing to bare the fact that there is a high click trough rate (CTR) on their ads, that comes from fraud clicks because eventually it pays off. Some advertisers offer this method because they believe it would be easier to attract affiliates to their program.

A special note is that this method ususally comes with another method- what we call "hybrid model" (pay per click + revenue share)

Pay per click can also appear in "sponsored link" – the owner pays the search engine to put his link high above other links. However, it does not guarantee, that surfers would choose to click on link; therefore it is not the best way to advertise.