From this wikipedia entry:
The JDK is a subset of what is loosely defined as a software development kit (SDK) in the general sense. In the descriptions which accompany their recent releases for Java SE, EE, and ME, Sun acknowledge that under their terminology, the JDK forms the subset of the SDK which is responsible for the writing and running of Java programs. The remainder of the SDK is composed of extra software, such as Application Servers, Debuggers, and Documentation.
The "extra software" seems to be Glassfish, MySQL, and NetBeans. This page gives a comparison of the various packages you can get for the Java EE SDK.
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.
Best Answer
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 (likejavadoc
andjdb
). 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.