JDK = Java Development Kit. It's the version of Java that you're coding.
SDK = Software Development Kit. Used for different Java technologies. e.g. Web Services SDK, Wireless SDK
JPad Pro = probably some text editor
JBuilder = a Java IDE, which has an editor, compiler, and a bunch of other tools built in
public classes are what you'll do most often. In any program you write, you can create an instance of this class. private classes are helper classes, and can only be used by the classes that are in the same source file as the private class, or classes in the same package if you've defined packages.