How is Java's "byte code" different from other codes (like source code, native code, etc.)?
Java's byte code is what the typical Java compilers create in the .class files. It is a binary language that is defined for the Java Virtual Machine (JVM). The JVM is the abstract machine which runs the byte codes (analogously to how an Intel 80386 runs .obj files).
The Java byte codes (and .class file format) are defined in The Java Virtual Machine Specification Second Edition by Tim Lindholm and Frank Yellin.