Core Java Understanding.

There are three main things in Java.

1. The JVM: This is the Java Virtual Machine. Now the deal is that this is an abstract concept.
2. The JRE: This is the actual implementation of the JVM. You thus have a JRE for Windows, Linux, Mac etc.
3. The JDK: This is the JRE + other stuff needed to program in Java. For instance, there is the Java Complier, the Debugger, the Profiler etc.