InterUser Interference in Web Apps?

[fripper]

I have a very basic question for which I cannot find a straightforward
answer in the .Net help files. I have a VB .Net web app that at one point
brings up a form (call it Form A) on which the user can enter a user ID
number and a password. Assuming the password is OK I then save the ID in a
global variable which is declared in a module. After viewing this form
(possibly taking some minutes to do so) the user can click on a button which
brings up another form (Form B) and using the ID saved in the global
variable some data relevant to this user is displayed. Now, my question is
what happens if a second user comes in and enters his/her ID number and
password in Form A and goes on to Form B while the first user is still
perusing Form A. When the first user finally goes to Form B will the global
variable still contain his/her ID and not that of the second user? I guess
that what this comes down to is does each user have a separate instance of
the program on the server ... in order to guarantee that one user cannot
affect another? I would think this could be very inefficient. What
principle is at work here? If someone can direct me to a discussion that
talks about this sort of thing I would greatly appreciate it.

Thanks very much.