1. The user interface, which runs on the user's computer (the client).

2. The functional modules that actually process data. This middle tier runs on a server and is often called the application server.

3. A database management system (DBMS) that stores the data required by the middle tier. This tier runs on a second server called the database server.

The three-tier design has many advantages over traditional two-tier or single-tier designs, the chief ones being:

Client/Server Software Architectures - An Overview
Provides a summary of some common client/server architectures and also summarizes mainframe and file sharing architectures. Links to more detailed descriptions for many of the individual architectures are provided throughout the document

Crossware and 3-tier Applications
Explains why 3-tier applications are a desirable evolution for client/server development in the enterprise.