A JAVA file is a source code file written in the Java programming language, which was originally developed by Sun Microsystems but is now maintained by Oracle. It uses an object-oriented approach, where structured data types, called classes, are used to instantiate objects at runtime.
JAVA source code files are compiled into .CLASS files using a Java compiler (javac command). A CLASS file contains bytecode that can be executed by the Java Virtual Machine (JVM). The JVM can be downloaded for every major operating system, including Windows, Mac OS X, and Linux.
NOTE: Android programs are written in Java but are run using the Dalvik Virtual Machine (DVM), which is a different virtual machine than the standard JVM that is optimized for Android.