To run Java, you need Java Virtual Machine (JVM or Java Run Enviornment jre). To write Java programs you would need Java 2 (j2ee or jese), which also includes a JVM.

Mac OSX comes with Java 2. No download is needed for running or writing Java programs.

Windows users may need to download a JVM (jre) or Java Plug-in for running Java programs. Download is avaiable at http://java.sun.com/getjava/ . Java 2 can be downloaded at href=http://java.sun.com/j2se/.