How Google Play services work
When Google previewed Google Play services at Google I/O 2012, it said that the platform "consists of a services component that runs on the device and a thin client library that you package with your app" (https://developers.google.com/events/io/2012/).
This means that Google Play services work thanks to two main components:
Google Play services client library: The Google Play services client library includes interfaces to each Google service used by your app. The library is included when you pack your app and it allows your users to authorize the app with access to these services using their credentials. The client library is upgraded from time to time by Google, adding new features and services. You may upgrade the library in your app through an update to your app, although this is not necessary if you are not including any of the new features.
Google Play services APK: The Google Play services APK runs as a background service in the Android operating system...
