Java is a programming language originally developed by Sun Microsystems in the mid 1990's.
Java applications can run on any Java virtual machine (JVM) regardless of computer architecture or OS, so a Java application can be written once and will run on Windows, Mac, Linux etc. without any recompilation. The JVM (or Java run-time environment, JRE) is the OS-specific component, not the application.
As to why you can't re-install it - have you fully uninstalled an existing version? What have you done that requires a re-installation? Are you trying to upgrade it? What OS are you using?
SD