As for as i know the use of environment variables in qtp 
is .
 in qtp the variable which you have in one action cant be 
used in some other action .so by using environment 
variables we can use the variables in one action in to 
another action

environment variables are 3 types,

1. user defined internal
2. user defined external
3. builtin

There are three types of environment variables: 

User-defined internal. Variables that you define within the 
test. They are saved with the test and accessible only 
within the test in which they were defined.

User-defined external. Variables that you predefined in the 
active external environment variables file. These variables 
are read-only in this context. 

Built-in. Variables that represent information about the 
test and the computer on which the test is run, such as 
Test path and Operating system. These variables are 
accessible from all tests, and are designated as read-only

I Agree with Umasankar. 

Built In Variables are activated at runtime only. by using
this variables we can know test execution information. That
is which user is executed, how Iterations were executed,
which Actions were executed, which test was executed, test
directory path..like that.

User Defined Internals..we can create them and these
variables are used only a test where they were created. Once
u created that variable u can not modify the name of
variable and u can modify the value. If u want to use these
variable in many tests u can export them in XML file. there
u can modify the name of the variable.

User defined Externals...The variables which are Imported
from XML file are called External variables. These are read
only variables. u can modify name and value of these
variables. If u want to modify them..go to XML file which
are stored and right click on that file click n edit. the
script will open, there u can edit name and value.

    this is completely about Environment variables.

If any queries..
nanda.dreddy@gmail.com