The
information
in an information system and the
technology
are completely tied together.

In fact, the one and only point of an info system is to
move
information from the people who
have
it to those who
need
it.

Each of the tiers can be seen as a way to
move
information around the loop.

The
presentation
tier is there to
output
information and also as a portal to
input
information into the system.
The
processing
tier is there to
transform
info from its form in the database and to check or
validate
info before it is stored.
The
data
tier, of course, is completely and totally about the information in the system.