Cost per lead campaigns pay you for each action that an end user completes on the advertiser’s site. Each advertiser defines this action differently.

An example of a lead would be a user filling out a form to receive free sample of a new product.

With cost-per-lead ads, you are paid for every unique person you send from your site/newsletter to the advertiser’s site if that person completes a specific action at the advertiser’s site.

The difference between is lead and a sale is a lead is a potential customer and a sale is a customer.