Java Runtime Environment (JRE) is Java’s most basic virtual machine. It’s a required component to run any Java-based application or web applet.  
                                                                                                                               
Java software allows you to run applications called "applets" that are written in the Java programming language. These applets allow you to have a much richer experience online than simply interacting with static HTML pages.

Java Plug-in technology, included as part of the Java 2 Runtime Environment, Standard Edition (JRE), establishes a connection between popular browsers and the Java platform. Java allows applications to be downloaded over a network and run within a guarded sandbox. Security restrictions are easily imposed on the sandbox.