First, I would recommend reviewing the pdf files that I have attached to this post.  The Multi-Tier document is a diagram of how the system is deployed in a macro scale.  Now, don't be frightened into thinking that you need separate servers for all of these things, some things can be combined into the same server.  However, we do recommend that the data servers are separate from everything else.  The Deployed Hardware Requirements has some more useful info as well. 

In short, most of the actual processing is done on the Application Business Logic Server and the MSMQ Server, however there is a "shell" application that resides on the client machines.  This is a .Net Framework application that accesses the various processes run on the servers.  The main benefit is that it is only one application as opposed to the 6 or 7 that exist with the Spectrum application.  One other large improvement over Spectrum from an IT perspective is that after the initial install, updating the application on the workstations are all "no-touch" deployments.  Any time we issue an update to the software, you decide when to apply those updates to the Application Host Server and the Data Servers, then the next time a client logs into the system it is automatically updated to the latest .dll files on the AHS.  No more running around to every workstation to copy the .mde files!