What is "android-billing"

Google Play In-app Billing is a form of selling items within the Android app. When we say in-app Billing on Android, what we mean is the sale of virtual items and services from an app. Since Android’s in-app Billing tools were released, their use has been steadily growing: more and more apps are implementing flexible monetization models, including virtual items and service subscriptions.

Reference: http://codigo-google.blogspot.com.br/2013/02/implementar-in-app-billing-em-android.html