NetSpades is a client/server application so in order to play across the network, spadesd must be running somewhere. This can be your local computer or a remote computer. Only one server needs to be running for you to play, and a single server can handle several games (5 by default).
Note that spadesd does not need to be running to play in off-line mode.
spadesd requires the use of two TCP ports. By default, ports 7626 and 7627 are used, but these may be overridden on the command line.
spadesd was designed to run continuously. If you are done playing (and are sure that no one else is playing!), you may stop the server with Ctrl-C. spadesd does not need to run as root.