Precisazione, forse superflua, ma che spesso mi capita di fare.
Per tier si intendono livelli "fisici", mentre per layer si intendono livelli "logici".
Citando da un paio di post:
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.
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.
Questa precisazione è necessaria anche perché spesso, soprattutto in ambiente universitario, si sentono usare questi termini a casaccio.