Android: No one likes ads, but they can get particularly creepy when you're not sure how an ad knew you might be interested in something in the first place. If you'd like your ads a little on the less-personal side, you can disable interest-based ads in the Google Settings app.
One of the most relevant for the privacy-minded is the ability to block interest-based ads. Open the Google Settings app from your app drawer, tap Ads, and select "Opt-out of Interest-based ads". This will block your phone from receiving tailored ad recommendations across your device.
It is against Google's developer policy to ignore this setting, so if an ad is still giving you targeted ads, that app could possibly be removed from the Play Store. You're on your own for any apps you downloaded from third-party sources, though.
This is also a good time to reset your advertising ID on the same screen. This ID is where all those targeted ads come from. So, if you'd rather Google not know that you're interested in that Hello Kitty adult product, you can reset your advertising ID and Google will start over from scratch.