What does APK stand for? What is the internal format? Is APK an abbreviation for Android Package Kit? What about the file itself? Is it  an archive such as ZIP where during install the installer extracts and copies the files onto the file system?
http://en.wikipedia.org/wiki/APK_(file_format)

APK files are a variant of the JAR file format, which are built on the ZIP file format, with a .apk file extension.