N-tier architectures usually have at least three separate logical parts, each located on a separate physical server. Each part is responsible for specific functionality.

When using a layered design approach, a layer is deployed on a tier if more than one service or application is dependent on the functionality exposed by the layer.

An example of the N-tier/3-tier style is a typical financial Web application where security is important: the business layer must be deployed behind a firewall, which forces the presentation layer onto a separate tier in the network.

Each tier is completely independent of all other tiers except those immediately above and below it. The nth tier only has to know how to handle a request from the n+1th tier, how to forward it to the n−1th tier (if there is one), and how to handle the results. Communication between tiers is typically asynchronous, to support better scalability.