Android has become very popular. And all the applications that are installable on android OS are supplied as APK files.
What's an apk? Single line answer is "APK is an android package file. i.e.; it just like an exe file for Windows OS and .rpm file for Linux OS."
APK file consists of all the files required to be run on android OS. i.e.; it contains image files and fully compiled Java classes with few configuration files.
APK file is a JAR based archived file with ".apk" as an extension.