is called an "n-tier architecture").

* Presentation tier
** This is the topmost level of the application. The presentation tier displays information related to such services as browsing merchandise, purchasing and shopping cart contents. It communicates with other tiers by which it puts out the results to the browser/client tier and all other tiers in the network. It is a layer that users can access directly (such as a web page, or an operating system's GUI).
* Application tier (business logic, logic tier, or middle tier)