Amazon Associates

You generate a tracked link per product. If somebody buys through it you earn a percentage of the sale, and the cookie window is 24 hours, so within that window you also earn on anything else that person buys, well beyond the item you linked. Rates are set by product category and mostly sit in low single digits. That makes this a volume game. Amazon's operating agreement is strict about how links may be used and how the relationship must be disclosed, and accounts do get closed for breaches. Disclose to your audience as well, plainly and before the link, which is a legal requirement in most countries.