A Non-Player Character (or NPC) is a character in Stellar Dawn that is not controlled by a player; rather they are controlled by the game itself. The functions of a NPC may vary; examples include monsters to fight, shopkeepers, or just random people around the world whom you can have a chat with (though NPC dialogue is always very similar). In RuneScape, they are represented on the minimap by a yellow dot. Non-player characters will probably be represented on a minimap in a similar way in Stellar Dawn as well.
Any non-player-character in RuneScape that can be attacked is classified as a monster. Many monsters drop item(s), trigger an event, or advance you in a mission, when killed. In RuneScape monsters are killed because they drop rare or valuable items. This may become true in Stellar Dawn as well.