Interest-based advertising lets Google act like a one-company ad network. It doesn't really need to know what other companies know about you because it can use information it gathers itself to show you targeted ads on any site that uses Google ads.
That isn't to say it doesn't get information from other companies, but that's not the only way it gathers it. Fortunately, you can turn interest-based advertising off. We'll also discuss a feature Google added that lets advertisers use your email address to target you.
To start, go to https://www.google.com/settings/ads. You'll see a different screen depending on whether or not you're signed in to a Google account.
If you're signed in
If you're signed in, you'll see a list of the interests or topics Google has collected, and you can deselect the ones that aren't really interests of yours, or that you don't want to see period. You can also add topics if you want.
Maybe a friend told you that you had to watch a great new video, but it turns out to be about snakes, which you hate. Google doesn't know that and might have added "reptiles" to your interests, which would explain the ads that contain snakes, lizards and iguanas that keep popping up. This is a quick way to stop that.
If you don't want to see interest-based ads, you can turn them off completely. Just set "Ads Personalization" to "Off."
Once you opt to turn Ads Personalization off, a pop-up window appears so you can confirm your decision.
As you can see, Google included some helpful bullet points to explain why Ads Personalization is better for you. It does have a point that if you're going to see ads online, they might as well be for something that interests you. However, balanced against Google building a profile of your interests, most people are going to opt to turn them off.
However, this setting only applies to ads you see on Google sites when you're signed in. There are more steps that you need to take to opt out of personalized ads when you are not signed in.