Android developers today are being notified that they need to declare by Jan. 11, 2016, whether their applications have ads in them or not. This currently is a requirement only for apps that want to be included in Google's "Designed for Families" category in Google Play. But starting early next year all applications will have to declare whether they include advertising.
Where things start to get a little interesting is when you talk about what an ad is. For these purposes, Google's including "ads delivered through third-party ad networks, display ads, native ads, and/or banner ads." And for the most part we all know what those look like. Sponsored articles also fall under the ads umbrella, Google says, as do "ads within a feed" and other kinds of native ads.
And Google will be doing some checking on this. If you attempt to say that your app doesn't contain ads, Google runs a quick check of SDKs included in your app and will nudge you appropriately.
It'll be interesting to see how users take to this, and whether install rates drop because of it. The Android Central app, for example, doesn't really have what we'd consider to be ads. It occasionally has webviews with display ads, though (though that's not the norm), and there occasionally are forms of native content. But if you asked me whether our app "has ads," I'd probably say no — because it doesn't in the traditional sense. But by Google's standards here, it does.