Java is a high-level object-oriented programming language created in 1995, developed by Sun Microsystems. Sun Microsystems has been bought by Oracle and since then Oracle is tightly coupling their databases and other products with Java. It is a vast open source language and offers more than any other language in terms of functionality and richness of its core and libraries. Java is an easy language to learn but difficult to master. It was designed to improve on the shortcomings on C++. With millions of Java programmers worldwide, it is clearly the most widely used programming language today.
Java code needs to be compiled
Languages such as PHP and Perl do not have compilers. They are executed line by line by interpreters. Languages such as C, C++, and Java are compiled. Java source code is compiled with a compiler producing bytecode. Bytecode runs on a virtual machine or run-time engine. C and C++ are compiled directly to binary code so they are much faster than PHP, Perl, or Java. Since bytecode runs on a virtual machine, there is additional overhead making Java unsuitable for heavy computation applications.
JDK, JRE, Java SE, Java EE
Java Development Kit (JDK) freely available at http://www.oracle.com/technetwork/java/index.html is the Java compiler. Java Runtime Environment (JRE) is the virtual machine. Code compiled with JDK run on JRE. Java Standard Edition (Java SE) is a JDK which provides tools and libraries for application development. Java Enterprise Edition (Java EE) includes all tools and libraries available in Java SE and also includes server-side tools and libraries essential for enterprise client-server applications. Both include and open source database management system, Java DB.
Here you will find some articles on Java.