This Google Opinion Rewards will give you an inside look and show exactly how it works. Google Opinion Rewards is a legit survey app you can use both for Android and iOS. The one I showed to you is an an Android app.

Get paid for answering short surveys. Download Google Opinion Rewards now: https://googleopinionrewards.page.link/share

Basically, the app offers very short surveys for a decent pay. You need to be aware that in some countries the only payout method is by using the credits you earn in the Google Play Store example like in Singapore. 

​Affiliate marketing is the process by which an affiliate earns a commission for marketing either a product or service from another company or partner.

When the affiliates promotes that product or service, he or she will earn a reward (certain amount of commission) from each sale that he or she make.
These sales are tracked via an affiliate links that was given to them.