In a pay per lead agreement, the advertiser only pays for leads generated at their destination site. No payment is made for visitors who don’t sign up.
A lead is generally a signup involving contact information and perhaps some demographic information; it is typically a non-cash conversion event. A lead may consist of as little as an email address, or it may involve a detailed form covering multiple pages.
One risk to the advertiser is the potential for fraudulent activity by incentivized 3rd-parties or marketing partners. Some false leads are easy to spot. Nonetheless, it is advisable to make a regular audit of the results.