Overview of a Java virtual machine (JVM) architecture. Source code is compiled to Java bytecode, which is verified, interpreted or JIT-compiled for the native architecture. The Java APIs and JVM together make up the Java Runtime Environment (JRE).
http://en.wikipedia.org/wiki/Java_Virtual_Machine
The CLR converts CIL (Common Intermediate Language) to native code.
http://en.wikipedia.org/wiki/Common_Language_Runtime