What is Java

1) a platform
2) a programing language
3) a ’virtual machine’ - a program (’run-time environment’) you can install on any device
that can run any programs written in Java. Those programs will be safely ring-fenced so that nothing they do can affect the working of the rest of your computer.

It can run code on pcs,mobile and other electrical devices (when compiled)

Where is Java used:

Basic web pages are plain text and coding to tell your browser how to display them.

Normal web pages may have graphics to make the page more attractive. They may also have audio and video clips.

Very complex web pages that change in response to user input.
These programs are typically written in either Java or JavaScript because all browsers support them.
So a lot of applications/sites will not work without Java.

If you encounter a website with an embedded Java app, and you don't have Java installed (or enabled),
you'll just see an empty space where the program (applet) should be displaying.

Alternatives to Java

Html5 is another development tools which is gaining in popularity