If you are a publisher using a cpc ad network on your website, and over time you figure out which ads are fetching you money, then you can get a general sense of what kind of products/services are of interest to your visitors. If your website is very specific, then chances are that you already know this. If not, then the top ads that are generating the revenue gives a good sense of what works better.
Once you have this idea, you can start doing research on the related websites and figure out if they also have affiliate programs. If they do, then you can sign-up for it and start promoting them on your website along side your advertisements. Then, if your user goes to that website through your affiliate link and end up taking the action, you end up getting paid a lot more than what an ad click would give you.
If you reach a stage that you have managed to identify the right set of affiliate programs, then you can completely ignore advertising altogether, but that probably is not as easy. So, a mix of both would probably a good idea atleast during the first few months when you experiment this. Also, not all companies have affiliate programs.
