Actually there exists tons of new applications that run over web pages environments, and one of the most common programming languages is Java. In order for the applications to be executed from the web pages, the internet browser needs complementary elements like plug-ins and libraries.
The J2SE(TM) Runtime Environment (JRE) helps developers and vendors so their Java applications can run because it contains the Java virtual machine,
runtime class libraries, and Java application launcher, components that are
necessary to run programs written in the Java programming language.
JRE does not contains the development tools so it is only used to run the Java programs, in order to develop programs the J2SE Development Kit is needed instead. JRE5 is free, easy to install and needs no configuration; it can be installed on Solaris, Windows and Linux ( for the 32bits version and 64bits version). After the program is installed all you will see will be the icon of the JRE5 application in the active application bar located on the left side of the task bar. JRE5 runs automatically when it is called by the Java programs that needs it.