There is no such thing as a free lunch, or so they say. Basically the idea is that if something is “free”, chances are there will be a hidden cost which is not necessarily monetary. The same can be thought of when it comes to free apps. Now there are some apps that are free without any in-app purchases or ads, but there are also a good majority of free apps that are based on ads.
This is only to be expected as the developers do need to get paid after all. However if you’d like to avoid apps with ads at all costs, Google might have a solution for you. According to the Android Developer page, it seems that come 2016, Google will require developers to declare whether or not their apps will contain ads.
According to Google’s definition, this includes banner and interstitials; native ads; and house ads (basically ads to promote the developer’s other apps). This does not include paid product placements or offers for in-app purchases or upgrades. The end result is that apps with ads will show that they do contain ads on their page, as you can see in the screenshot above.
Both Apple and Google have been making strides in trying to make their app stores as transparent as possible. For example “free” apps are no longer listed as free on the iTunes App Store, and apps with in-app purchases are also clearly stated so parents can decide if this is a game they want their kids to play, lest they rack up a huge credit card bill.