Well, I'd think there is no clear standard generally approved difference between "tiered" and "layered" - eveybody has their own standards :)
Have a look: http://en.wikipedia.org/wiki/Multitier_architecture
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'Three-tier' is a client-server architecture in which the user interface, functional process logic ("business rules"), computer data storage and data access are developed and maintained as independent modules, most often on separate platforms. The term "three-tier" or "three-layer", as well as the concept of multitier architectures, seems to have originated within Rational Software[citation needed].
The three-tier model is considered to be a software architecture and a software design pattern.
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