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A tier is a group of runtime components that comprise a computing system. A tiered architecture is one that separates the concerns various parts of a computing system into different executable parts. For example, a 3-tier architecture will have a component that executes the user interface, a tier for business logic, and a tier for data services.
While 'tiers' are similar to layers, they are really a way to split the 'execution architecture' rather than the build time architecture. This is not to say, that a layered architecture can execute each layer in a tier. For some layered systems this makes sense. For others (eg: a network protocol handler) this doesn't make sense.
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