What does a just-in-time (JIT) compiler do?

Question

What does a JIT compiler specifically do as opposed to a non-JIT compiler? Can someone give a succinct and easy to understand description?

Answer 1

A non-JIT compiler takes source code and transforms it into machine specific byte code at compile time. A JIT compiler takes machine agnostic byte code that was generated at compile time and transforms it into machine specific byte code at run time. The JIT compiler that Java uses is what allows a single binary to run on a multitude of platforms without modification.