Multi-tier architecture (often referred to as n-tier architecture)
is a client–server architecture
in which the presentation, the application processing, and the data management
are logically separate processes.
For example,
an application that uses middleware
to service data requests between a user and a database
employs multi-tier architecture.
The most widespread use of multi-tier architecture
is the three-tier architecture.
N-tier application architecture provides
a model for developers to create a flexible and reusable application.
By breaking up an application into tiers,
developers only have to modify or add a specific layer,
rather than have to rewrite the entire application over.
There should be
a presentation tier, a business or data access tier, and a data tier.
Sumber:
http://en.wikipedia.org/wiki/Multitier_architecture