These are large advertising networks that pay you everytime someone clicks on a ad. Depending on the network, pay per click network ads are based on keywords chosen by the webmaster or are automatically targeted based on the keyword content of each page. The clickers do not have to actually buy or do anything else after they have clicked on your ad, in order for you to be paid. Click fraud is common with these networks and you may get banned from some of these networks, even if you are not guilty of anything.