<P> Three - tier architecture: </P> <Dl> <Dt> Presentation tier </Dt> <Dd> 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 . In simple terms, it is a layer which users can access directly (such as a web page, or an operating system's GUI). </Dd> <Dt> Application tier (business logic, logic tier, or middle tier) </Dt> <Dd> The logical tier is pulled out from the presentation tier and, as its own layer, it controls an application's functionality by performing detailed processing . </Dd> <Dt> Data tier </Dt> <Dd> The data tier includes the data persistence mechanisms (database servers, file shares, etc .) and the data access layer that encapsulates the persistence mechanisms and exposes the data . The data access layer should provide an API to the application tier that exposes methods of managing the stored data without exposing or creating dependencies on the data storage mechanisms . Avoiding dependencies on the storage mechanisms allows for updates or changes without the application tier clients being affected by or even aware of the change . As with the separation of any tier, there are costs for implementation and often costs to performance in exchange for improved scalability and maintainability . </Dd> </Dl> <Dd> 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 . In simple terms, it is a layer which users can access directly (such as a web page, or an operating system's GUI). </Dd> <Dt> Application tier (business logic, logic tier, or middle tier) </Dt>

Web concepts- architecture of 2 tier or 3 tier web application