Java is an object oriented language created by Sun Microsystems in the mid-nineties to provide an open and machine independant platform to develop applications on. The original driving force behind Java was the need to write application code for web applications that could be downloaded via a browser and then executed on a client, regardless of its operating system, be it Windows, Mac, Unix, Symbian or anything else.
Java has since grown extensively as more uses have been found for such a versatile language and more platforms have been brought into existence. Thousands of different constructs (for example "collections" and "frameworks") have been created and built into the Java language itself, making the language extremely adaptable. Java has even been used to write entire operating systems.
The key part of Java that allows it to be run on on such a diverse range of hardware and operating systems is the JVM, or Java Virtual Machine. This is in effect a program that acts as a virtual computer which can understand Java; Java programs are then run on this virtual machine.
Java tends to occur in the mobile telecommunications industry in two common forms - J2ME (Java 2 Micro Edition), which is the set of Java objects and interfaces available on mobile platforms such as phones; and downloadable Java applications (most commonly Java games) which are developed by software houses and then installable over the air by any handset user.
The definitive resource on Java is at java.sun.com