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

20% of the byte code is used 80% of the time. The JIT compiler gets these stats and optimizes this 20% of the byte code to run faster by adding inline methods, removal of unused locks etc and also creating the bytecode specific to that machine. I am quoting from this article, I found it was handy. http://java.dzone.com/articles/just-time-compiler-jit-hotspot