s3d is build as client-server architecture, similar to the xserver. The s3d-server is the program executed on the users desktop and waits for incoming connections, the applications, which give their content they want to have displayed to the server. The server will then display the content to the user, who can move to the 3D space, and interact (mouse moving/clicking, keyboard), and the client will receive these signals over the network from the server.
Just like on the X-Server, it may seem unusual to have the "server" on the own machine, while the "client" may be on any other machine with network connection. But this makes features possible like browsing files on your file server or even access point. ;)
Don't let the server-client thing confuse you too much for now, on a properly installed system the s3d will startup with the mcp (which is the window manager in s3d-terms) which will give you a complete look and feel just as you would expect.
More (internal) information about the server will be presented here as soon as someone is interested. ;)