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.
The Java 2 Runtime Environment can NOT be distributed as part of Debian as it would violate Sun's license for the software. One of the requirements for redistributing the J2RE is that you not distribute any other software intended to supersede the functionality of the J2RE. As Debian distributes free projects like Kaffe, we can not distribute the JRE.
On Microsoft Windows platforms, but not in Linux, 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.