The Java Development Kit is the compiler, JVM, tools and all the libraries and APIs defined as "Java 2 Standard Edition" or J2SE. The "extended edition" or J2EE adds servlets and EJBs and a number of other APIs. Some of these features have implementation classes and some are just interfaces and standards that any vendor may implement.
Sun also provides "reference implementations" of some of the APIs in J2EE, like a servlet container and an EJB container. They often say the reference implementations give you an idea of how things work, but may or may not be production ready. This is a neat way of doing business as it lets other vendors compete on high capacity implementations of a solid standard and provides a baseline for inter-operability.
WSAD is one of many development tools. It's based on Eclipse (which you can download for free) and adds features (that IBM thinks are worth a lot of money) to generate and manage J2EE applications among other things. There are open source plugins that compete with some of these features.
A good question is never answered. It is not a bolt to be tightened into place but a seed to be planted and to bear more seed toward the hope of greening the landscape of the idea. John Ciardi
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Building a Better World in your Backyard by Paul Wheaton and Shawn Klassen-Koop