APK is a format specifically for Android devices. It uses a .apk extension and can be downloaded via Android software sources.

[edit] The format

This file format is a variant of the JAR format and is used for the distribution and installation of bundled components onto the Android mobile device platform.

An APK file is a ZIP file, with a particular internal file layout that allows it to be run in place, without unpacking. This format usually consists of 2 folders and several files. This includes:

• META-INF (folder)
• res (folder)
• AndroidManifest.xml
• classes.dex - a compiled Dalvik Executable.
• resources.arsc

[edit] For more information

• http://en.wikipedia.org/wiki/APK_(file_format)