Ok, this is for more advanced Excel users. Here's the issue.

Lets say I have two sheets in Excel, S1 and S2 respectively. Let's say in
S1, if I type a 4-digit number IN COLUMN A, which is actually an ID number
for a person, I want their name to automatically appear in COLUMN B from a
list in S2.

This is how S2's list is set up... A1 has a person's ID #, and B1 has that
same persons name. A2 has a different person's ID #, and B2 has that person's
name, etc... So when I type the ID # in S1 in column A, it will lookup in S2
and match that person's name with their ID.

I know this may be confusing, but I can't think of any way else to word it.
Any help will be very appreciated. Looking forward to your comments. Thank
you!!