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Fat clients do prolific processing.
Thin ones are more like dumb terminals that just repeat whatever the server tells them. Each tier is a sort of checkpoint for moving data. Or you could compare it to a manager with solid communication skills and a penchant for delegating duties. Application servers essentially translate raw data from, say, a company's inventory or finance database, into information with real world meaning relayed to and displayed on a desktop or Web browser. There's a bit of confusion in the application-server product category because the technology and terminology are still relatively new.