A reference to a JavaPackage object that represents the top node of the java.* package hierarchy.
A reference to a JavaPackage object that represents the top node of the netscape.* package hierarchy.
A reference to a JavaPackage object that represents the top node of the sun.* package hierarchy.
The Packages JavaPackage object contains three properties--java, sun, and netscape--all of which refer to other JavaPackage objects representing the java.*, sun.*, and netscape.* Java packages.
The Window object also contains "global" properties named java, netscape, and sun, all of which are synonyms for the properties of the Packages object. So instead of writing Packages.java.lang.Math, for example, you can just write java.lang.Math.