A server machine manages and stores the shared data in an application. In a networked system, this includes the database, the system database and the DP4 utilities.
Any workstation can act as a server. There are no special characteristics, other than that the machine is connected to the network and has sufficient disk space for the system and database files.
The server machine can be configured as a dedicated server, or as a workstation that runs the server in the background. As a dedicated server, the display can show you which users are logged onto the system.