This is a question that is often asked but seldom answered fully. In this website you will hopefully find answers to the majority of your affiliate related questions.
Affiliates can be a source of revenue in any of the following ways:
a Clickbank affiliate - Clickbank has thousands of affiliates who normally pay you a commission for the sales of their digitally downloadable products oe ebooks
Lead generation - many companies will pay you for leads that you generate for them to sell their products. Quite often insurance or mortgage finance.
You can sell a physical product on behalf of another company and get paid commission - you don't have to stock or even ship the product.
The above are just a few examples of affiliate marketing and as you digest the details in this website you will undoubtedly find many ways of getting an affiliate.