Java 2 Platform, Standard Edition (J2SE) platform for applications development on desktops and servers. It also serves as the foundation for J2EE and JWS.
J2SE consists of the J2SE class library, JVM (platform specific byte code interpretor), and other development tools and APIs (which are shipped with JDK).
The J2SE application programming interface (API) defines the prescribed manner by which an applet or application can make requests to and use the functionality available in the compiled J2SE class libraries
The JRE provides the Java APIs, Java virtual machine, and other components necessary to run applets and applications written in the Java programming language. And this JRE is also the foundation of the J2EE runtime environment.
JDK = JRE + Development tools