The single-player (SP) side of Neverwinter Nights is where a person plays the game without the possibility of others joining. Neither an internet nor LAN connection is necessary, or even checked for (aside from verification of premium modules, if one is being played). Most modules can be played in this mode, in part because it is easier to make a module work for single-player than for multiplayer.
A single-player session is started by clicking "Play" from the opening (splash) menu (or by running nwmain.exe directly), then selecting either "New" (to start a new game) or "Load" (to load a previously-saved game). When starting a new game, the next window provides choices for starting one of the official campaigns, going to a list of premium modules (if any have been bought), or going to a list of the other modules present on the computer. This is followed by a choice to go to character creation or to use an existing player character, either one pregenerated by BioWare or one that had been earlier created on that computer.
During a single-player game, the player has the ability to pause the game (with the space bar, by default) and has full access to DebugMode and console commands. (In multiplayer games, these are typically reserved for the host.)