The software or applications you run in any device are written by programmers.
The process of writing instructions that run in a computer is called computer programming. The CPU(Central Processing Unit), runs these instructions one by one.
However, it only understands the instructions in form of machine language.Machine language comprises only zeros(0s) and ones(1s).
Our minds, however, cannot recall and work with thousands or millions of 0s and 1s. So experienced software engineers have created programming languages in high level languages that more understandable to us. These are more english-like than just 1s adn 0s.
Then a special program called compiler then compiles these instructions to machine form so that they run in the CPU. One of these high level languages is Java.
Java is a high-level programming language that follows in the tradition of C/C++. Sun Microsystems, the original company that created Java described it as "a simple, robust, object-oriented, platform-independent, multithreaded, dynamic, general-purpose programming environment".
James Gosling and Patrick Naughton led a group of other software engineers at Sun Microsystems to create a programming language that was code-named "Green". This was in 1991.
The idea was to use this in consumer devices like intelligent televisions, ovens and referigerator. However, 3 years later, the market was still quiet.
One year later, in 1995, at SunWorld exhibitions, Java was introduced alongside a browser that could run applets(java applications that can run in the browser).
Since then it has achieved phenomenal rate of growth and is now one of the most popular languages for everything form programming games, creating mobile applications to creating high performacne trading applications used in Wall Street etc.
As a general purpose language, Java can be used in almost all industries for almost any type of application.
Java as a language is Object Oriented. This allows you to relate your program constructs to real world objects. This provides a better mental model to think and design our programs. Besides we get the advantage of reusability, which is one of the primary advantages of this paradigm.
Java is an adaptable language. And at the core of this adaptability, and hence popularity, is its platform independent nature. This stems from the way Java itself operates and in particular from the use of an abstract execution environment. This environment allows the Java code to be separated from the underlying operating system.
NOTE: Sun Microsystems, the original creator of Java is now a subsidiary of Oracle Corporation. It was acquired in the year 2010.