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Click Bank vs Commision Junction

Clickbank only lists services and products for sale, while Commission Junction lists websites, shops, services, software, and anything in which money is involved. There are different types of offers on CJ like CPM, CPA. But Clickbank is only a CPM.
What's CPM means? Sorry because I'm new in affiliate field. So which one you think more profitable? I try commission junction but haven't generate any sales yet :( heard people talk a lot about Clickbank saying that it's easy to get money with them, but still doubt to apply there because it's not free...