Advertising someone else's products is almost same as putting Adsense on your websites or blogs. There is one difference however you get paid if someone buys that product through your website. You get paid lots to do it. Some websites offer upto 15% in comissions if you sell a product through your website.
It is very challenging to sell a product through your website unless you dedicate it to products. For example your website is all about games then you can manage to sell playstation, xbox consoles or games through your website.
You have to see what visitors to your site are looking for and what people generally buy online. People don't generally buy clothes online so don't waste your time advertising clothes, foods, etc.