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May 25, 2012 06:06 AM|LINK
A tier is a physical separation i.e. different hardware for different purpose.For ex: your Database server is on another machine while Business Logic is not another server.
A layer is a logical separation i.e. same hardware for different purpose.For ex: your UI is on a machine while Business Logic is in another set of classes.
This would further explain things:
A n-tier architecture is a physical structuring mechanism, while a n-layer architecture is a logical structuring mechanism.
While is true, for example, that a 3-tier application is (at least) a 3-layer application, a 3-layer application could have only 1 or 2 tier(s).