JRE
(Java Runtime Environment)

Computer users who run applets and applications written using Java technology
An environment required to run applets and applications written using the Java programming language
It is an implementation of the Java Virtual Machine* which actually executes Java programs.
Java Runtime Environment is a plug-in needed for running java programs.
The JRE is smaller than the JDK so it needs less Disk space.
It includes the JVM , Core libraries and other additional components to run applications and applets written in Java.

Java SE
(Java Platform, Standard Edition)

Software developers who write applets and applications using Java technology
A software development kit used to write applets and applications using the Java programming language

JDK
(Java Development Kit)
It is a bundle of software that you can use to develop Java based applications.
Java Development Kit is needed for developing java applications.
The JDK needs more Disk space as it contains the JRE along with various development tools.
It includes the JRE, set of API classes, Java compiler, Webstart and additional files needed to write Java applets and applications.