What is Dalvik Executable (DEX)?

• Compiled Android application code file. 
• A optimized format for efficient storage. 
• All classes are compiled with a standard Java language compiler. 
• A tool called dx is used to convert some (but not all) Java .class files into the .dex format. Multiple classes are included in a single .dex file. Duplicate strings and other constants used in multiple class files are included only once in the .dex output to conserve space.