N-tier and n-layer are entirely different concepts. People often use this term interchangeably during the design of the application architecture. N-tier refers to the actual n system components of your application. For ex: Suppose you create a web application, its components include your application server where it is being hosted, the database being used with it, on another server and the user machine who access the application, forms the third component. These are referred to as the 3 tiers of your application. So they may be n in number and so the term n-tier application. So tiers are the physically separate components of the same system. On the other hand, layers refer to the internal architecture of your component. For ex: in your code, you divide the code into different layers like Data access layer, Business logic layer etc.. So they are internal to the component and these layers interact with each other internally to form the entire component.

N refers to # of tiers or # of layers in ArchitectureLayers 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 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.Tiers 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.

http://stackoverflow.com/questions/11542960/main-difference-between-3-tier-n-tier-architecture-in-net