Per sign up program pays you a flat rate every time that person sign ups. All programs claim that they pay per trial membership as well. Another option is per click. In this case, its very simple, they offer a rate, simply multiply by the number of hits. The only important aspect of this option is to find out if they pay per click on the second page or whatever the case might be.
Some programs offer the blind click program and thats a click to front page counted.
Third option, and the program used by the most sponsors is partnership program. In this case, you could be earning percentage of revenues for every sign up. That can vary between 50 percent up to 70 percent in some programs. The plus side to this option is the reccuring revenue. Your next step would be to pick one of these options.