Listen Server
From GMod Wiki
Definition
A "listen server" is an instance of Garry's Mod running as both a "client" and a "server" at the same time. This means a client has started a server (by using "Create Server" in the main menu or "map <mapname>" in the developer's console) to host a session and is also running a client (his personal session) of Garry's Mode simultaneously. Listen servers are used by users who play in single player mode, or players who wish to host a game with other players in multiplayer modes without a separate computer to do so.
See Also
Player.IsListenServerHost- A function to determine if the current player is the host of a listen server.
Dedicated Server- a server run on a machine used specifically for running a Garry's Mod multi-player server.