Java Runtime Environment - If you don't plan to create ["Java"] apps yourself, then this is enough.
The Runtime Environment for Linux contains the Java Virtual Machine (["JVM"]), Java core classes, and supporting files. It does not contain any of the development tools (such as appletviewer or javac) or classes that pertain only to a development environment.
The Java 2 Runtime Environment is smaller than the Java 2 SDK
The Sun's Java 2 Runtime Environment is redistributable, unlike the Java 2 SDK. The Java 2 Runtime Environment License ( http://java.sun.com/j2se/1.4.2/j2re-1_4_2_06-license.txt ) allows you to package it with your software. By distributing the Java 2 Runtime Environment with your application, you can ensure that your customers will have the correct version of the runtime environment for running your software.
On Microsoft Windows platforms, the Sun's Java 2 Runtime Environment's installer automatically installs the java and javaw application launchers in a location that's on the operating system's default system path. That means you don't have to worry about finding the launchers to start your application, and you don't have to provide instructions to your users for fiddling with the paths on their systems.
- For creating Java apps, ["SDK"]