The 3 types of advertising
CPM - Cost per thousand views (from the latin M=1000)
CPC - Cost per click, the most common form of advertising, payment for each unique ip which clicks on the advert.
CPA - Cost per action, payment for each successfully completed purchase from the advertiser's website, usually commission based.
Which pays best?
Although this question has been the subject of many debates it totally depends. For the advertiser, CPA guarantees they only pay you (the publisher) when you generate a sale, so they cannot lose. In return, you might get 25% commission, so if you sell a product costing $200, you will get $50, which is a lot for just sending one visitor their way. However, CPC results in much more regular payments, and I rely on CPC for the majority of my website income. Finally there is CPM - CPM has become used less and less frequently due to the possibilities of click tracking with modern web languages, it now makes it much easier to generate stats using CPC models. However, with CPM, the advertiser knows upfront roughly how much he will be spending (based on previous page view statistics for the publisher site).
I recommend CPC and CPA, I combine the two as I have found they pay better than CPM and offer better visitor tracking.
Good luck!Copyright notice
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