Android Applications are primarily written in Java programming language. The Java source files are converted to java class files by the java compiler.

The Android SDK Contains a tool called dx which converts Java files to .dex (Dalvik Executable) file. All files are compressed in one .dex redundant information in class file are optimized in .dex file.

dex file are smaller in size than class files. The dex file resources of an Android project are packed in .apk (Android Package) file. aapt (Android Asset Packaging Tool) performs this packaging.

Via the adb tool the necessary data in .apk file is run and deployed to an Android device.

The ADT (Android Development Tools) performs these steps transparently to the user. If you use the ADT tooling you press a button and a whole Android Application is created and deployed.