If we analyze history, we will come up with three types of Database Application Architecture are as follows:

Single Tier

In Single Tier or Mainframe architecture everything from database to application program used to be in single computer.All processing occurs on this single computer. The user interacts with application through dumb terminals.

Two Tier

In Two Tier or Client/Server architecture, database reside in Main Computer called server and Application program reside in each user computers called Client.Processing is divided between two, data processing occurs at Server and application processing is the responsibility of the client machine.

Multi Tier

In Multi Tier or Nth Tier architecture,there are no specific numbers of tier. It assumes that a program is comprises of different types of components and each component has part to play. According to the resources needed by each components we need to decide whether to put it in another machine or not. All the Tier can be in same machine or different machines depending on it use.

Forms 6i, 9i and 10g are the example of such architecture where we have components such as Database, Application Server, Web Server, Forms Services, Report Services and so on. These component can be installed in one machine or many depending on the usage and size of the system.

Same way Web Database Application is another example of such architecture.