A non-player character, or NPC for short, is a character in the game that is not player controlled (in other words, is computer controlled). In Backyard Monsters, the NPCs are called the Wild Monster Tribes. Some examples of Wild Monster Tribes are the Dreadnaut Tribe, the Abunakki Tribe, the Legionnaire Tribe, the Kozu Tribe and the new Moloch Tribe (Inferno) and Brukkarg Tribe (event).
Note that although all of your monsters are also non-controllable when attacking, you still control where to fling them in and when to Putty Rage them, etc., and therefore, monsters are not NPCs.
The NPCs are servers that are set to automaticlly attack, repair, etc.