They really differ in use. Many operating systems come in several flavors because of this, e.g. Windows, Linux, etc. A Desktop OS targets an end-user, and is [usually] optimized for Graphical Interfaces, Media, et al., while not needing robust features required or needed by Server OSs. Server OSs are meant to be used by multiple users and as such are optimized for background processing, application/file sharing, print services, databases, internet and network services, fault tolerance, etc. Network OSs are meant to be used by clients (such as a Terminal Server Client for Windows or VNC or an X-Windows client in the case of Unix.) Applications and ilk run on the Network OS across the wire, the client just sees a representation of it on their "terminal." The client is usually the only software running on the local machine when using a Network OS -- and may be a very stripped down OS whose sole purpose is to be a "terminal" to a Network OS.
Ian Mariano - Bliki | Blog
"We are all wave equations in the information matrix of the universe" - me