Difference Between Jdk and Jre

What is the difference between JDK and JRE?

The JRE is the Java Runtime Environment. It is a package of everything necessary to run a compiled Java program, including the Java Virtual Machine (JVM), the Java Class Library, the java command, and other infrastructure. However, it cannot be used to create new programs.

The JDK is the Java Development Kit, the full-featured SDK for Java. It has everything the JRE has, but also the compiler (javac) and tools (like javadoc and jdb). It is capable of creating and compiling programs.

Usually, if you only care about running Java programs on computer you will only install the JRE. It's all you need. On the other hand, if you are planning to do some Java programming, you need to install the JDK instead.

Sometimes, even if you are not planning to do any Java development on a computer, you still need the JDK installed. For example, if you are deploying a web application with JSP, you are technically just running Java programs inside the application server. Why would you need the JDK then? Because the application server will convert JSP into Java servlets and needs to use the JDK to compile the servlets. I am sure that there are more examples.

What is the difference between JVM, JDK, JRE & OpenJDK?

JVM

The Java Virtual Machine (JVM) is the virtual machine that runs the Java bytecodes. The JVM doesn't understand Java source code; that's why you need compile your *.java files to obtain *.class files that contain the bytecodes understood by the JVM. It's also the entity that allows Java to be a "portable language" (write once, run anywhere). Indeed, there are specific implementations of the JVM for different systems (Windows, Linux, macOS, see the Wikipedia list), the aim is that with the same bytecodes they all give the same results.

JDK and JRE

To explain the difference between JDK and JRE, the best is to read the Oracle documentation and consult the diagram:

Java Runtime Environment (JRE)
The Java Runtime Environment (JRE) provides the libraries, the Java Virtual Machine, and other components to run applets and applications written in the Java programming language. In addition, two key deployment technologies are part of the JRE: Java Plug-in, which enables applets to run in popular browsers; and Java Web Start, which deploys standalone applications over a network. It is also the foundation for the technologies in the Java 2 Platform, Enterprise Edition (J2EE) for enterprise software development and deployment. The JRE does not contain tools and utilities such as compilers or debuggers for developing applets and applications.
Java Development Kit (JDK)
The JDK is a superset of the JRE, and contains everything that is in the JRE, plus tools such as the compilers and debuggers necessary for developing applets and applications.

Note that Oracle is not the only one to provide JDKs.

OpenJDK

OpenJDK is an open-source implementation of the JDK and the base for the Oracle JDK. There is almost no difference between the Oracle JDK and the OpenJDK.

The differences are stated in this blog:

Q: What is the difference between the source code found in the OpenJDK repository, and the code you use to build the Oracle JDK?
A: It is very close - our build process for Oracle JDK releases builds on OpenJDK 7 by adding just a couple of pieces, like the deployment code, which includes Oracle's implementation of the Java Plugin and Java WebStart, as well as some closed source third party components like a graphics rasterizer, some open source third party components, like Rhino, and a few bits and pieces here and there, like additional documentation or third party fonts. Moving forward, our intent is to open source all pieces of the Oracle JDK except those that we consider commercial features such as JRockit Mission Control (not yet available in Oracle JDK), and replace encumbered third party components with open source alternatives to achieve closer parity between the code bases.

Update for JDK 11

An article from Donald Smith try to disambiguate the difference between Oracle JDK and Oracle's OpenJDK : https://blogs.oracle.com/java-platform-group/oracle-jdk-releases-for-java-11-and-later

As mentioned in comments by @Alan Evangelista, Java Web Start has been deprecated by Oracle in Java SE 9 and removed in Java SE 11.

Java SE 6 vs. JRE 1.6 vs. JDK 1.6 - What do these mean?

When you type "java -version", you see three version numbers - the java version (on mine, that's "1.6.0_07"), the Java SE Runtime Environment version ("build 1.6.0_07-b06"), and the HotSpot version (on mine, that's "build 10.0-b23, mixed mode"). I suspect the "11.0" you are seeing is the HotSpot version.

Update: HotSpot is (or used to be, now they seem to use it to mean the whole VM) the just-in-time compiler that is built in to the Java Virtual Machine. God only knows why Sun gives it a separate version number.

My understanding about JDK, JRE and JVM

JVM is virtual, you can think of it as a virtual computer whose machine code is the Java bytecode. It is not a real computer but a virtual one which can execute Java bytecode instructions.

JRE is the JVM implementation. The implementation is OS-specific (of course) but it provides OS-independent outer interface (meaning e.g. you can run same Java code on a Windows JRE and on a Linux JRE). This is the 'write-once-run-anywhere' thing from the late 90s.

JDK is the compiler, the JRE, other tools, and all Java APIs you need to write Java code.

I don't want to add Android and its Dalvik VM to the picture (though I can draw some interesting parallels), as I will confuse you more, I think.

JDK/JRE/JVM/Java SDK | What do they all mean? Sometimes you can develop with JRE and sometimes you need JDK?

Eclipse has its own built-in compiler (called ecj), which is probably the reason you could get away with not having the JDK installed to use it. It does not use javac.

Google App Engine uses the javac that comes with the JDK.

What is the difference between the JRE and JVM?

The JRE is the environment within which the virtual machine runs.

JRE - JAVA Runtime Environment

JVM - JAVA Virtual Machine

JRE is the container, JVM is the content.