You need a Java Development Kit (JDK) or a Java Runtime Environment (JRE) on your system to run Magnolia and the Tomcat application server that ships with the bundle. JRE is an implementation of the Java Virtual Machine which executes Java programs. JDK is a bundle of software that you can use to develop Java based applications. If you plan to write your own Java code get the JDK. If you just want to run Magnolia then JRE is sufficient.