From: Lloyd Taylor <Lloyd.Taylor@jhuapl.edu>

A three-tier architecture introduces a server (or an "agent") between
the client and the server.  The role of the agent is manyfold.  It can
provide translation services (as in adapting a legacy application on a
mainframe to a client/server environment), metering services (as in
acting as a transaction monitor to limit the number of simultaneous
requests to a given server), or intellegent agent services (as in
mapping a request to a number of different servers, collating the
results, and returning a single response to the client.

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