JIT compiler - Nice Idea

JIT Compiler: 

Taking the best of two worlds - results in interpreting(converting to machine code) machine independent byte code at the cost of  dynamic compilation (generates optimized code based on target machine) + caches translated code to minimize performance degradation

JDK vs JRE

JDK: The JDK includes the compiler and other tools needed to develop Java applications

JRE: JRE is a subset of JDK(light weight), it contains java.exe which is used to run an already compiled java application. Unlike JDK, JRE does not include java tools like compiler or other tools. 

JDK vs JRE: JDK is a superset of JRE, it can be used to develop java applications, compiling and running java applications. JRE is just for running already compiled application.