It defines some built in values like NaN( not a number) , Infinity ( infinitely large number) and undefined .
Contains eval(), parseInt() etc as function properties.
The most important properties are the Constructor properties
Object(), Function(), Array(), String(), Boolean(), Number(), Regexp(), Error() etc. they perform different depending on use like as a function and as constructor.
the frequently used Math and JSON object is also the property of Global Object.
Will post an another detailed post on implemention this week.