Java is a powerful, cross-platform, object-oriented programming language suitable for writing anything from a distributed application that runs on a corporate network to a database-driven Web site to host your personal photo gallery.
The Java Runtime Environment (JRE) provides the libraries, the Java Virtual Machine, and other components to run applets and applications written in the Java programming language. The Java SE application programming interface (API) defines the manner by which an applet or application can make requests to and use the functionality available in the compiled Java SE class libraries.
The Java Development Kit (JDK) forms an extended subset of a software development kit (SDK). In the descriptions, which accompany their recent releases for Java SE, EE, and ME, Sun acknowledges that under its terminology, the JDK forms the subset of the SDK, which is responsible for the writing and running of Java programs.