Java Virtual Machine - JVM

When a Java program is
compiled it is converted to byte code which is then executed by the Java
interpreter by translating the byte code into machine instructions. Java
interpreter is part of Java runtime environment.  Byte code is an intermediate code independent
of any machine and any operating system. 
Program in Java run time environment, which is used to interpret byte
code, is called Java Virtual Machine (JVM). 
The Java compiler reads Java language source files, translates the
source into Java byte codes, and places the byte codes into class files. 

Any machine for which Java
interpreter is available can execute this byte code. That’s why Java is called Machine
independent and Architecture neutral. Below Figure shows that Java
compiler is accepting a Java program and producing its byte code. This byte code
can be executed on any operating system (Window-98, Macintosh, Linux etc.) running
on any machine with suitable Java interpreter of that machine. The JVM plays
the main role to making Java portable. It provides a layer of abstraction
between the compiled Java program and the hardware platform and operating system. The JVM is
central to Java’s portability because compiled Java programs run on the JVM,
independent of whatever hardware is used.

Java Virtual Machine