Affiliate programs are commonly misunderstood, in order to understand affiliate programs lets start with terminology. For clarification purposes, an affiliate is defined as any "referrer" or website that promotes a product in an effort to earn revenue. A merchant is defined as someone who owns a product and is sharing revenues with an affiliate based on the affiliate's performance. Affiliate programs can drive targeted traffic to your website.
There are 3 basic affiliate programs, though only the first two are commonly used.

Pay Per Click - This is when an affiliate is compensated for sending traffic to the merchant. (AdSense is an example of PPC affiliate program)

Pay Per Sale - This is when the affiliate is compensated by the merchant if the referral generates a sale or purchase.

Pay Per Lead - This is when the merchant agrees to pay for a qualified (or sometimes unqualified lead), which is very uncommon because it is subjective and up to the merchant.

Affiliate websites tend to provide information, entertainment, and content services to their customers. The online merchants sell products, goods and services online. These are programs permitting affiliates to earn money based on the visitors to your site who click through to another's website. Some pay a token amount for the click through and others provide a percentage of sales when a visitor "clicks through" to your site and buys a product or service on the other party's site. This could represent a value added service to your visitors.