You can deploy your app in 1-, 2-, 3- and (with Silverlight) 4-tier physical configurations.
In a 2-tier model, the data is remote because the client uses the database communication model to talk to the database server.
In a 3-tier model, the client uses some network protocol/technology to talk to an application server. Usually this is WCF, but it can be Remoting, asmx web services or Enterprise Services (DCOM), or a custom channel.
Obviously in the 3-tier model something has to host the code on the app server. This is often IIS, but can be WAS or a custom Windows service.
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