Bash Shell Environment

[mss]

Ok, so from what I understand, there are three types of environment var.

Shell variable (defined globally and loaded at the system boot, /etc/profile)
user-defined variable (defined local for the user as he/she logs in, ~/bash_profile)
special variable. (I HAVE NO IDEA)

So here is my question, if a variable is defined in as a shell variable (/etc/profile), and the same variable is defined in user-defined variable, which one will take effect?

And also any ideas on what the special variables are and how they are defined.

ps. one last question, if we're adding a user to the system how can we set his user-defined variables. ie (is there a template for the user-defined variables that can be editted by the root so that all new users have the new change already made)

[iwanabeguru]

1.
The user defined variables will take effect

3.
if you want automatic settings for user variable for evry new user , make a
.bash_profile and copy it to /etc/skel.