Environment variables are two types
1) Built in
2)User Defined

User Defined also two types
1)User Defined Internal
2)User Defined Internal

Environment Variables are variable whose value is fixed for 
its usage everywhere.
There are two types.
1.Built in-which are already existing in QTP which we can 
directly use in out script.
ex:ActionName,ActionIteration

2.User-defined
which user has to define and use those variables anywhere 
in the action.
Ex:File path-Suppose u r reffering a data file in ur script 
then store the location of ur file path in environment 
variable and use it across the actions.

Built in Enironment 
a=Environment.Value("TestName") 
msgbox a   (this gives test script name )
if u see in qtp Resources Environment built in u find many 
 things like TestName, Pathdir,version etc

User defined Evironment

i want to store a value in run time and pass to another
script then
a=Enironment.Value("DocumentNumber")