From Wiki: N-tier:
In
software engineering, 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" refers to three-tier architecture.
Single tier:
Single-tier architecture is the
notion of having the user interface of a web application, the middle ware and the data access all contained in one whole architecture.
Usually in
Software Engineering you break your application to make them more maintainable and sustainable.
If you know that your software will not grow over time, you may want to do a single tier architecture.
Two tier:
A
two-way interaction in a
client/server environment, in which the user interface is stored in the client and the data are stored in the server. The application logic can be in either the client or the server.
Three-tier:
Refer my answer
3tier application using c#[
^]
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