What is the difference between 3 Tier & 3 Layer Applications ?

The terms tier and layer are frequently used interchangeably, but actually there is a difference between them:

Tiers indicate a physical separation of components, which may mean different assemblies such as DLL, EXE etc on the same server or multiple servers; but layers refers to a logical separation of components, such as having distinct namespaces and classes for the Database Access Layer (DAL), Business Logic Layer (BLL) and User Interface Layer (UIL).

Therefore, tier is about physical separation and units of deployment, and layers are about logical separation and units of design.