The Android App package is an .apk file that contains all the necessary information to install the application on your android phone. It is a Java class file and a resource file, which together contain the application binary.
An application program is first compiled, and then its components (Java classes) are packaged into one single file to make it an .apk file. This file is usually compressed in zip format. With some tools, such as WinZip or WinRAR, it can be opened.
When you download an app and install it, you are actually installing an APK file, which is an archive file. It's very easy to install an APK file from a file manager app or directly from a desktop. But, for security reasons, the ability to install from a desktop or file manager app is disabled. In order to enable it, you must change the setting of 'Unknown sources' from the settings.
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