Web Applications can be organized in many different ways. I use the “three-tier” model for its simplicity, ease of maintenance, and modular scalability (different parts of an application can be scaled up separately).

Here is what the three tiers are and what they do:

	Presentation Tier		Viewing Controlling
	Business Tier		Doing Business Transactions Executing Workflow
	Data Tier		Getting Data Storing Data

Each Tier can also be associated with common technologies:

	Presentation Tier		Web Browser Web Server
	Business Tier		Web Services Business Logic Components
	Data Tier		ODBC Connections DBMS

Tiers are built on one another. The presentation tier sits on the business tier which sits on the data tier.

Here’s the path data follows when is transferred and transformed from the data tier to the presentation tier. That occurs when a user requests a web page, for example.

Data Tier

Business Tier

Presentation Tier

It could translate into the following implementation:

Database

ODBC Connection

Web Service

Web Server

Web Browser

Of course, when a user sends information to a Web Application, by filling out a registration form, for example, data inversly follows the same logical path.

Data Tier

Business Tier

Presentation Tier

... which could translate into:

Database

ODBC Connection

Web Service

Web Server

Web Browser