Software Architect's Handbook

Tiers versus layers

You may have heard the terms tier and layer in reference to layered architectures. Before we proceed with discussing layered architectures, these terms should be clarified.

Layers are logical separations of a software application and tiers are physical ones.

When partitioning application logic, layers are a way to organize functionality and components. For example, in a three-layered architecture, the logic may be separated into presentation, business, and data layers. When a software architecture is organized into more than one layer, it is known as a multi-layer architecture. Different layers do not necessarily have to be located on different physical machines. It is possible to have multiple layers on the same machine. ...