The commission structure of a program determines what action your affiliates will be paid out on. Commissions are most commonly paid out on a sale, but can also be set up to be paid for a lead/sign up.
Each advertiser has differing margins and goals for their affiliate channel and it is recommended to work out the CPA of customers via other paid channels and running a cost comparison to ensure ROI is met, and affiliates are remunerated fairly for their efforts.
Post-view tracking, or view-through tracking, is a small window where a commission payout can be assigned to an affiliate after a customer has been served an impression. View-through tracking is commonly used for content affiliates and bloggers.