N-tier application architecture provides a model for developers so they can create a flexible and reusable application. Because an application is broken down into tiers, developers only have to modify or add a specific layer, rather than rewriting the entire application over if they decide to upgrade or change technologies. In the term "N-tier," "N" implies any number (like 2-tier, or 4-tier or any number of tiers used in your architecture). Application architectures are part of Layer 7 of the OSI reference model.

For more about application architecture, see this article: http://www.15seconds.com/issue/011023.htm

For a diagram, see this page: picture:http://www.webopedia.com/img/app.arch.gif