I see a lot of the same questions being asked here over and over so I will try to cover the basics of working with affiliate programs. Please note that these are my opinions that have worked for me on different sites in different categories so you might not get the same results, however, these should work for most sites.
What are the different types of affiliate programs
CPC - Cost Per Click
Cost per click is just as like it sounds, you get a certain amount of money for each click that is generated from your site.
CPM - Cost Per 1,000 Impressions
This means that you will be paid a certain amount for every 1,000 banner impressions (views) displayed on your site. Some affiliates will pay you based on raw impressions which means that every singe banner will get you paid no matter if its from the same user viewing the same ad on different pages or not. Other programs will pay you based on 1,000 unique impressions which means that 1 impression counts from 1 visitor per 24 hour period (this is the standard but some programs have different time periods).
CPA - Cost Per Auction
CPA means that you will be paid a commission on sale or lead generated from your site. The commission is based on the program you sign up for.
Which is right for me?
This is a question that no one can answer for you. You will need to do some research on your visitors to find out which will work best. On sites with smaller traffic CPA usually works better while CPM and CPC will perform better on larger sites. My advice is you try each type of ad to see which performs better on your site.
When using any of the above you should always find a program that will interest your visitors. For example, a site about cars probably won't get very good performance displaying ads for cosmetics. This might seem like common sense, however, you would be amazed. Sometimes what you think your visitors will be interested in is completely wrong or you could find something that performs better. I have seen people that put up a product not really related to the site but they got very good results. This will again require research on your part. Setting up polls to see what your visitors are interested in is always a very good idea. Also, note that visitor interests will change over time because of different sites linking to you and your site appearing on different keywords on search engines.
When starting to work with any affiliate program you should have some kind of a visitor base. Before spending a lot of your energy on resources spend some time developing your site and promoting it so it gets some kind of visitors. The number of visitors you should have on a site before starting any type of affiliate program all depends on the content of the site. Some sites will get very good return on 10,000 visitors a day while other sites can get the same return on 50 visitors a day.
Popular Programs
Again, I can not tell you which programs will work for you based on the above logic, however, here a few programs that can work for many sites.
http://www.google.com/adsense/
This is a rather new program from google that will display targeted banners on your site and pay you on a CPC basis. What's nice about this program is that every banner is extremely targeted toward your visitors so you will get a good click through ratio and a good rate on your clicks.
http://www.cj.com
Commission junction has been around for a long time. They are probably the largest affiliate program out there and can help you find a advertiser that will work on your site. The key here is to find an advertiser that will interest your visitors as everything is on a CPA basis.
There are also hundreds of thousands of other affiliate programs that might work for you. Try searching google for a product related to your visitors, once you find something see if they have an affiliate program you can join. If they don't try contacting the company, many times they will be happy to do business with you or make you an offer to display their ads on your site.
Never stop looking for a program that could make you money simply because you are already making money. You should always try new things to see how they perform and after you have tried a few different programs stick with the one that works best. Feel free to post any questions you may have here and I will try to help you the best I can, however, please don't PM or email me personally as I will probably ignore the message.
Good Luck,
Paul