Logical layers are merely a way of organizing your code. Typical layers include Presentation, Business, and Data – the same as the traditional 3-tier model. But when we’re talking about layers, we’re only talking about the logical organization of code. In no way is it implied that these layers might run on different computers or in different processes on a single computer or even in a single process on a single computer. All we are doing is discussing a way of organizing a code into a set of layers defined by specific function.Physical tiers, however, are only about where the code runs. Specifically, tiers are places where layers are deployed and where layers run. In other words, tiers are the physical deployment of layers.

Tier is a physical separation of application and layer stands for logical separation. If we divide our application in different parts i.e Data layer , Business logic and User interface i.e layer. If we host our application parts (DAL,BL,UI) in different location or server i.e tier.

Tier is a physically separated component running on different machines or environments.Layer could be logical separated component.-------------------------------------------------------------------------------------------- 3 tier architecture comprises of 3 physical tiers of application environment. A. Presentation Tier-: Basically, all your UI code runs here. HTML, CSS, ASPX etc. End user access to this tier. B. Business Logic + Data Access Tier-: Component classes, web services, data access classes etc C. Data Tier-: Database hosted here is here on independent server. ----------------------------------------------------------------------------------------------- 3-Layer -: 3 Layer is pattern to structure your application similar to above mentioned components but it is not mandatory that all layers will run on physically different tiers.