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.