The Extras package available in the SDK Manager are libraries made available by Google to facilitate the development of applications that use some of their API, such as Google Maps, Google Billing, and others.
Completing the answer:
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Android Support Repository: So, like Android Support Libray, this library supports the development of unique features. Android Support Libray Is a library that provides classes that facilitate the development of some functionalities, such as working with Fragments in android. Good libraries of this package and more used are Android Support V4 and Android Support V7 AppCompat. Android Support v7 AppComppat supports the use of ActionBar and other features available for Android versions above 3.0, but this library makes it possible to use that in Android 2.1 (API 7). You can check out here .
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Google play services for Froyo: This library supports the use of the Maps API V2 among others like Google Cloud Messaging itself. It fits both Froyo and other versions of Android. There is a specific version of this library for Froyo because in version 2.2 (API 8) of Android there is a flaw in the geolocation issue among other peculiarities. So if you're going to develop an app that runs on this version of Android and you're going to use the Google Play Services library, use that version of the library.
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Google Repository: For this, see if this link helps you.
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Google play APK Expansion Library: This library is heavily used in game features. You can see a very thorough explanation of this library on these links: link1 , link2
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Google play Billing Library: With this library you can make your application charge. If you want to sell a feature within your app, you can use this library. It has a range of features that validate buyers' purchase and license.
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Google play Licensing Library: With this library, you can validate the license of users who have purchased your app in the Play Store. In this case, the app is sold in the Play Store and has no purchase capability within it.
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Google Web Drive: See if that link helps you.
As I said above, Google Cloud Messaging is available in the Google Play Services library.
Here are some examples of how to add and use some extras.
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