Please refrain from multi-posting.
<http://www.physci.org/codes/javafaq.jsp#xpost>

It consists of the classes by which the application obtains and modifies
its data.  If the database itself is considered part of one of the tiers
then it is in the third tier, but the database is not the focus -- in
fact, the application is poorly designed if it is discernible above the
lowest layer that a database is in use at all.

The tier names vary, but I find these more descriptive:

1) Presentation
2) Business Logic
3) Persistence

You could as well call the third layer "Data management", but it
probably isn't conducive to a good mental model to call it "Data_base_
management".

No, you have a misapprehension.  The persistence (a.k.a data management)
layer provides everything that the business logic layer needs to access
and modify business data; all interfaces and API for this purpose are
part of the persistence layer.  If an underlying database needed to be
changed (in a compatible way) then the data access interfaces and API
would NOT need to change, nor would any higher layer -- only the private
implementation of the persistence layer would change.  There is no extra
interface in between layers 2 and 3.  If there were, then that would
make it a 4-tier architecture.

A key here is that each layer should depend only on the layer below it
(where that means that the persistence layer depends only on the data
representation).  The interfaces exposed by each layer should be kept as
stable as possible, but the internal implementation of each layer may be
more variable.

The database isnt (shouldnt?) be an integral part of the data managment
layer. This layer simply offers an abstract interface to manage data,
regardless of how or where it is stored. Basically you'd tell this layer to
modify or retrieve data in some way, and depending on the implementation of
the interface, it'd talk to the database, a text file, a spreadsheet etc. In
other words, it hides the details of how the data is stored and managed from
the rest of the application.

Remon van Vliet