In exchange, you agree to share information about yourself that Google can share with advertisers so their ads are more effective. For instance, airlines want to target people who love to travel. Children's clothing makers want to target parents.
Google uses a lot of methods to learn about you. There's the stuff you tell Google outright when you sign up for its Gmail, maps or to use your Android phone. This includes your name, phone number, location, and so on.
But Google also watches you as you scamper around the internet, deducing your interests from your internet searches (what do you search for? click on?), from your use of Google's other services, and from other websites you visit.
By visiting a hard-to-find page called "Web & App Activity" you can see what Google is watching.
Then by visiting a site called "Ads Settings" you can see what Google thinks it knows about you and you can change what its telling advertisers about you. Read on for the details...