You mean, how the Java Virtual Machine (JVM) interprets and/or compiles the class file into a running program? Head over to java.sun.com and search for "JVM"; that will bring up some articles and links that will get you started. If you are REALLY interested in the JVM, you might even study the JVM specification (also on java.sun.com).
I'm not dead! I feel happy! I'd like to go for a walk! I'll even read a tiny ad: