[_delirium]

You join an affiliate program that pays you some commission per lead or sale, like Amazon's that pays you 4%+ (depending on volume) per item sold, or many of the things on places like Commission Junction, which include things like 1% of airfare sold. Then you buy ads which point to your affiliate link; if your affiliate payouts are greater than the cost of the ads, on average, you make the difference.

Some affiliate programs (like Amazon) don't allow this. Others are happy to allow it because it sort of outsources the work of figuring out how much to pay for ads. Instead of a company having to decide what to pay for e.g. Google AdWords, they just decide what percentage royalties they want to pay out for leads, and let third parties figure out how to bid on AdWords in a way that doesn't exceed that cost.