<p>What's the difference between "Layers" and "Tiers"?</p>

Read Scott Hanselman's post on the issue:
A reminder on "Three/Multi Tier/Layer Architecture/Design"
:
Remember though, that in "Scott World" (which is hopefully your world also :) ) a "Tier" is a unit of deployment, while a "Layer" is a logical separation of responsibility within code.  You may say you have a "3-tier" system, but be running it on one laptop.  You may say your have a "3-layer" system, but have only ASP.NET pages that talk to a database.  There's power in precision, friends.