1. CPC:
This is the classic Cost per Click. Your ad spends will be based on per click by a visitor.
2. CPM:
Cost Per Mille. Your ad spend will be based on impressions. A single site visitor is equal to one impression. Per mille equal to 1000 visitors.
3. CPA:
Cost Per Action. Your ad spend will be based on user actions such as a sale, registration, file download or a survey. CPA is the most favorable ad by publishers compare to CPC and CPM.
4. POP:
Same as CPC, your ad spends will be based on per click by a visitor.