Is used to run certain applications/amimations on webpages. It's advisable to install it as updates often include security fixes etc, and generally run better. It should be either Runtime 5 update 10 or runtime installing. After the install is successful go to start>control panel>add or remove programs and look for Java entries in the list as I have found it often leaves old versions on taking up space. Anything earlier than Runtime 5 update 10 you can uninstall.