environment variable is a variable,Which you have in one
action can't be used in some other action so by using
environment variable we can use the variables in one action
in to another action.

These are global through variables

if i am wrong correct me plz

We can use the environment  variables for speed up the 
testing process. For example During testing if at all we 
have the long path, then we give the user defined variable 
to that path and run the programm.

It can be used to pass values across actions
syntax is Environment.value=""

The main use of Environment variable is the value of
environment variable does not change