How Machine Learning Algorithms Help Businesses Target Their Ads

It happens all the time. One minute, you're Googling, "what is a differential?" The next minute, you're looking at toasters on Amazon or exercise equipment on eBay, and all you see everywhere are pictures of axle girdles and Trac-Lot rebuild kits. If the internet is going to flood your senses with ads anyway, they might as well be ads for things you're interested in. But how do they do it?