Instructional Material: What is Java?
Java is a general purpose programming language developed in the mid 1990s by Sun Microsystems. Java is now owned by Oracle Corp. Java is a text based, object oriented, class based programming language (we will learn what these terms mean later). In addition to the language itself, Java is designed to allow a program to be written once on a selected hardware and operating system platform and then executed on different hardware and operating system platforms through the use of virtual machine technology. This means the language and the programs generated from that language are not platform (hardware/operating system) dependent. An example is writing Java robot programs on a Windows PC but executing the resulting programs on Linux based robot controllers. This idea is called "write once, run anywhere".
Here is the Wikipedia discussion of Java.
Note that Java is not the same thing as JavaScript. While these two programming languages share ideas and have similar syntax they are definitely different in their purpose, design and implementation.