The heart of the Java Platform is the concept of a 'virtual machine' that executes Java bytecode programs. This bytecode is the same, no matter what hardware or operating system the program is running under. There is a JIT (Just In Time) compiler within the Java Virtual Machine, or JVM. The JIT compiler translates the Java bytecode into native processor instructions at run-time and caches the native code in memory during execution.
The Java virtual machine is called "virtual" because it is an abstract computer defined by a specification. To run a Java program, you need a concrete implementation of the abstract specification.
Whenever a Java application starts, instance of JVM is created. JVM instance starts running the application by invoking main() method of some initial class.
Sun has claimed there are over 4.5 billion JVM enabled devices.
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