A JVM (Java Virtual Machine) is an imaginary (theoretical) machine that executes Java bytecodes.
A JRE (Java Runtime Environment) is a software implementation of a JVM and all the standard Java libraries, and other bits and pieces that are needed to make the thing actually work.
A JDK (Java Developer's Kit) or J2SDK (Java 2 Software Developer's Kit) are two different names for the same thing: a JRE plus tools like the Java compiler, debugger, and other things needed for basic Java development.
Somebody else has asked the same question today; I hope it wasn't you asking twice. Go to the API website, which you need to become familiar with, and click on the name of a package (if you have the frames view on, the list of packages seems to stay in the upper left frame).
If you want to see the actual text of the classes, you have to join the Sun Developers' Network.