If you describe someone as a monster, you mean that they are cruel, frightening, or evil. unusually large = giant
A monster is something which is extremely large, especially something which is difficult to manage or which is unpleasant. the monster which is now the London marathon
A bodybuilder of exceptionally large weight and proportions, typically weighing more than 135 kilos (300 lbs)
an alien being that looks scary; sometimes they're really big and mean - monsters live in higher dimensions where they can only be seen by children and other people who can use their third eye - if a monster is bothering you, ASK LEO FOR HELP and he'll take it away
whereas a monster will play various different creatures and NPCs as assigned by the GM, usually a new one for each encounter (although often something fairly generic such as 'a zombie' or 'a wolf') In most encounters, most of the monsters will end up dying or running away
The term depicts a larger range of creatures than usual: technically, any NPC is a monster - except that the more "usual" races (e g those available to the players) are mostly called "NPCs"