Once you have ported AOSP to your platform, an obvious next step is to install some apps: maybe Gmail or Angry Birds. Ideally, you would want Google Play Store so that you can select apps from the 100,000's available. But, there is a problem. Play Store, like most of the apps on Play Store, requires Google Mobile Services (GMS) to work. GMS extends the Android framework by adding the libraries, configuration files and applications required to support the extended Google APIs that most developers know simply as Google Services. Most of these components are in the system partition so either they have to be included as part of the base AOSP build or installed as root afterwards. There is also a licensing issue: the components that make up GMS and applications like Gmail, Google Maps and Google Now are proprietary to Google and require a license.