what is the difference b/w N-tier archicture and N-tier layer?

It's a common mistake to confuse between layer an tier.
Layer is a logical layer that part of the architectural design of an application. It talks about the logical separation of different parts of the application like UI, DAL...
Tier is a physical location where code run. For example you may have a application server to run DAL and a web server to run UI.

N-tier is basically the physical separation of your system components, where as N-layer is logical separation of your components of a system.
Tiers- A Tier within an application simply refers to actual physical separations within your application and where your code might run (and how it might actually be deployed). You can think of Tiers as the areas where your layers will be deployed.
System split in stages of processing. Tiers can be replaced by other implementations. Example: client/server is a two-tier system. We can also say that, tiers are the physical deployment of layers.

Layers- Layers act as more logical separators that exist to separate and organize your actual code. You'll often hear terms like "Business Logic Layer", "Presentation Layer" and others. These are simply ways to organize all of the code within your application.
System split in different levels of abstractions. Visualized as a stack.
e.g
1)Presentation Layer or UI Layer
2)Business Layer or Business Logic Layer
3)Data Access Layer or Data Layer