An open architecture allows the system to be connected easily to devices and programs made by other manufacturers. Open architectures use off-the-shelf components and conform to approved standards. A system with a closed architecture, on the other hand, is one whose design is proprietary, making it difficult to connect the system to other systems.

Also see a diagram of n-tier application architecture in the quick reference section of Webopedia.

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University of Wisconsin's WWW computer architecture page 
This resource page offers numerous links to computer architecture groups and projects, individuals and technical organizations, commercial information, literature, and publications.

Yahoo!'s page of computer architecture
Yahoo!'s directory of computer architecture.

Hardware Central
Detailed test reports on the latest desktop and laptop computers. Plus news on trend-setting products just coming into the pipeline.

Internal Processor Architecture and Operation
Describes processor instruction sets and modes, internal architectural components, the instruction execution process, and performance enhancing architecture features. This page is from "The PC Guide."