Sun's Java Home Page (http://www.java.sun.com/) describes Java as follows:
The Java platform is a fundamentally new way of computing, based on the power of networks and the idea that the same software should run on many different kinds of computers, consumer gadgets, and other devices.
Since Java's inception in 1995, it has quickly moved to the forefront of computing technology, supported on nearly every computer platform. The key to Java is the Java Virtual Machine, a layer of software that runs between the computer's main operating system--like Linux, OS/2 or Microsoft Windows--and a Java application or applet.
The Java Virtual Machine, or JVM for short, acts as an intermediary between the application and the host operating system: The application talks to the JVM, and the JVM talks to the operating system. The goal of Java is that software developers can make exactly one version of an application that talks to a compliant JVM. There will be, in turn, many kinds of JVM's that run on a myriad of computer platforms; but every JVM should talk to Java applications the same way.
In short, the JVM's function is as the universal software adapter that allows all Java software to run on any Java enabled computer, regardless of processor, operating system, or manufacturer. If you own more than one computer running different hardware or different operating systems, Java is a boon because you only need to buy one version of an application, and it should run on each computer. What if you only have one computer? How does Java benefit you then? The answer lies in the increased time and resources available to software developers to make their products better. If a company has to write one version of a product, they will spend less time duplicating functionality on different platforms and more time fixing bugs and adding features you desire to the product.