Clickbank is mostly information products - ebooks and software. You are paid a percentage of the sale price, the user downloads the product. Pros: Maybe some good products for some niches. Cons: A lot of crap, a lot of get rich quick scheme ebooks, no one here will like you if you are selling ebooks!
Copeac is mostly CPL & CPA - you are paid when the user submits their details or completes an action or sale. They generally pay a fixed amount for each lead or action. Amongst other things they have ring tones, financial and free ipod etc offers. Pros: Offers which convert well, they are regularly on WickedFire and more than anything they actually go out of their way to help you do well. (Makes sense, the more money the affiliates make, the more they make). They are also really friendly! Cons: There isn't that much choice compared to other networks, particularly if you are looking to promote outside the US.
It really depends on what you want to promote. If you don't know yet and want to start with a network that looks after it's affiliates try Copeac. You might as well sign up for both networks anyway (and others) and compare their products.
Hope that sort of makes sense....I really need sleep!.... or something like that.
I just have to passmy exams and ill start some serious business.