Layer is logical seperation of the system. Typical 3-layered architecture comprises Presentation (UI) layer, Application (business) logic layer and Data Access or persistence layer.
Tier is physical seperation of the system. If the 3 layers are deployed on the same machine, then we have 1 tier and 3 layers. If the 3 layers are deployed on three seperate machines, then we have 3 tiers and 3 layers.
