The App Store equivalent for the Windows Phone is the Android
Market which is available to Non Apple products.
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There are several apps for the Windows phone that aren't
available at the Windows store for PCs, including certain
games.
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The Google Play Store can't be downloaded on a Window's phone at
this time.
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Yes, the Windows 8 phone app store has many apps that you can
download, but Google Play comes pre-installed on your phone.
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There are many different places that sell mobile phone software
online. The main ones are the Play store (Android Phones) the Apple
Store (Iphones) and the Windows Phone Store.