What is the difference between JDK and JRE?

Question

What is the difference between JDK and JRE?
What are their roles and when should I use one or the other?

Answer 1

Simply :

JVM is the virtual machine Java code executes on

JRE is the environment (standard libraries and JVM) required to run Java applications

JDK is the JRE with developer tools and documentation

Answer 2

JDK is a superset of JRE, and contains everything that is in JRE, plus tools such as the compilers and debuggers necessary for developing applets and applications. JRE provides the libraries, the Java Virtual Machine (JVM), and other components to run applets and applications written in the Java programming language.