Layer and Tier: Layers refer to logical seperation of code. Logical layers help you organise your code better. For example an application can have the following layers. 1)Presentation Layer or UI Layer 2)Business Layer or Business Logic Layer 3)Data Access Layer or Data Layer The aboove three layers reside in their own projects, may be 3 projects or even more. When we compile the projects we get the respective layer DLL. So we have 3 DLL's now. Depending upon how we deploy our application, we may have 1 to 3 tiers. As we now have 3 DLL's, if we deploy all the DLL's on the same machine, then we have only 1 physical tier but 3 logical layers. If we choose to deploy each DLL on a seperate machine, then we have 3 tiers and 3 layers. So, Layers are a logical separation and Tiers are a physical separation. We can also say that, tiers are the physical deployment of layers.