To use a class that is part of a package you have to import that class. This is done with an import statement, usually placed near the beginning of the file in which the class is used. For instance, to use Vector we would place the following statement at the beginning of our file. In particular you should check out java.lang which contains Integer, Float, Double, Math, Object, String, StringBuffer, and System among other classes. The entire java.lang package is always imported automatically.
System is a class that is part of the java.lang package.
The java.lang package is always imported automatically so the class System is always available. The class System is basically a standard place to hold various items related to the system. In particular, System.out represents the standard output and println is a method of the file System.out. Methods For fields and methods, access is controlled by modifiers. A final method can't be overridden by a method of the same name declared in a subclass.