Java Development Kit (JDK)
The Java Development Kit is the SDK that provides the tools needed to build apps that can be run on the Java Runtime Environment (JRE). The JDK includes extra tools on top of the JRE to develop applications, while the the JRE is needed to run applications.
Java's class library is contained in rt.jar (or classes.jar for older Java versions on OS X) and has lots of types spread over several "packages", which in Elements are called namespaces. Java has its base classes (Object, String and the object wrappers for the basic types) in java.lang. Other interesting classes list lists and maps are in the java.util namespace.