JRE is an acronym that stands for Java Runtime Environment. This is a powerful software that installs Java libraries on your computer so that you are able to run applications written in Java on your computer without any difficulty. It also includes Java Plugin that makes it possible for apps to run inside your browser as well as Java Web Start that establishes Java applications in a network. You get this software officially from Sun Microsystems. If the Java system installed in your computer is older, you can face security issues as older versions do not have security updates. You can download and install JRE 1 5 Sun Microsystems here.