A monster is a creature usually hostile towards the Player Character. Monsters are the 'non-civilized' inhabitants of the universe, at least from the average city-dweller's point of view. Non-humanoid monsters often have no sense of culture or language.
In Neverwinter Nights, monsters also include extra planar beings, such as devils, demons, and elementals. These creatures often describe the PC races as monsters themselves, or were once themselves of the PC races.