Imagine you are trying to find out what the people in someone's department
are doing.
You know the name of the department head, and you want to look at
the home pages of members of his or her department.
The interactions between your browser and the web might look like this
(time proceeds downward):
The thread of control is resident in the browser (and the user), while the various servers that are accessed are essentially subroutines that each do a small part of the job.
Homepage 