Here's an explanation of set! and set-structure! in terms of what goes 
on in the computer's memory.  You won't be tested on this material --
these are just notes that may help some of you see the difference
between them.

When you make a structure, such as (make-account 3 100), Scheme makes
a place in computer memory to store the account and its data.  If you
then give that structure a name, Scheme associates that name with the
place (location) where it stored the data:

   Program:  (define acct1 (make-account 3 100))

   Memory:   acct1 -----------> [account, 3, 100] ;; a chunk of memory

If you put that defined name into another struct, Scheme makes another 
reference to that same location:

   Program:  (define acct1 (make-account 3 100))
             (define Mcust (make-customer "Maria" 5551234 acct1))

   Memory:   acct1 -----------> [account, 3, 100] ;; a chunk of memory
                                |
                                |----------------|
                                                 |
             Mcust -----------> [Maria, 5551234, |]

Set-structure! changes the contents of the structure (between the
square brackets in our picture), but not the associations between
names and locations.  So (set-account-balance! acct1 150) changes our
memory picture to look like

   Memory:   acct1 -----------> [account, 3, 150] ;; a chunk of memory
                                |
                                |----------------|
                                                 |
             Mcust -----------> [Maria, 5551234, |]

Note that the connections between data in memory are unchanged, so
changing acct1 causes the change to show up in acct1 (we'd see a
similar change if we did set-account-balance! on the account in Mcust.

Set!, in contrast breaks the association between names and locations.
If we now do (set! acct1 (make-account 4, 800), Scheme moves the arrow
from acct1 to refer to the new account:

   Memory:       | -----------> [account, 4 800]
                 |   
             acct1              [account, 3, 150] ;; a chunk of memory
                                |
                                |----------------|
                                                 |
             Mcust -----------> [Maria, 5551234, |]

As you can see, the set! did not affect the account in Mcust.  Further 
set-account-balance! operations on acct1 will not affect Mcust,
despite the original connections between the data.

Again, if this helps, great.  If not, not something you'll need to
know in detail.