People often use the two terms interchangably in that they can describe an architecture as being both multi-layered and multi-tiered. My take on it is that generally a tier refers to some physical separation while a layer is more of a logical separation.
For example, a typical web application I would say has a front end - what's displayed in the browser - and the actual application logic running on the application server, and a database. This could be referred to as 3 tiered, since there is a database server, an application server and the client machine. Just as easily, however, one may refer to the database layer, the logic layer and the presentation (or UI) layer.
Difference in layer and tier architecture[
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http://stackoverflow.com/questions/1182303/n-tiered-vs-n-layered-architecture-design[
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