JVM

Architecture : JDK contains JRE which inturn contains JVM

It is the principal component of Java architecture that provides the cross platform functionality and security to Java. This is a software process that converts the compiled Java byte code to machine code. Byte code is an intermediary language between Java source and the host system.

Most programming language like C and Pascal translate the source code into machine code for one specific type of machine as the machine language vary from system to system. So most complier produce code for a particular system but Java compiler produce code for a virtual machine. The translation is done in two steps. First the programs written in Java or the source code translated by Java compiler into byte code and after that the JVM converts the byte code into machine code for the computer one wants to run.
So the programs files written in Java are stored in .java files and the .java files are compiled by the Java compiler into byte code that are stored in .class file. The JVM later convert it into machine code. In fact the byte code format is same on all platforms as it runs in the same JVM and it is totally independent from the operating system and CPU architecture. JVM is a part of Java Run Time Environment that is required by every operating system requires a different JRE. JRE consists of a number of classes based on Java API and JVM, and without JRE, it is impossible to run Java. So its portability really made it possible in developing write once and run anywhere software.