.dex stands for Dalvik Executable Format. This is executable file which is developed by google. In android, if we compile our programs then it will generate .dex (Dalvik Executable) files, which are in turn zipped into a single .apk (Android Package) file on the device. .dex files created automatically when our programs are compiled and translate the compiled applications, written in the Java programming language.

It is similar to java class files, but they are run on older android version under the Dalvik Virtual Machine (DVM). If you want to decompile .dex file then use dexdump tool which is provided in android-sdk. It is a developer files that is used to initialize & executes our programs.