APK

APK is an archive format used for distributing Android apps. It is based on the Jar format (for Java), and like that format, is actually a ZIP archive with a different extension, and with specific files and directories within it.

Metadata about the app is in a META-INF directory within the archive (similarly to Jar files), compiled code is in a lib directory, and resources in a res directory. There are also a few files at the root level including AndroidManifest.xml (which may be regular XML or, often, binary XML).

Links

 * Wikipedia article