Creature is basically every living breathing (or undead) thing big enough to be considered at least CR0 (give or take).
It's important to note that "creature" does not get a precise definition in the rules. However, we can infer from BD&D p4 that it includes both the PCs and the creatures they encounter.
Sometimes the adventurers and other creatures do their best to kill or capture each other in combat.
Thus we can say with some certainty (and examining how the word is used throughout the text) that creature is the broadest definition of living thing. If a spell, effect or condition affects "creatures" then it effects everything it targets.
So in summary, when it says "creature", it means everyone, it if says "humanoid" it generally means one of those races. If it says "player" or "charater" or "player character" it means a player controlled character. If it says "non player character" it means a DM controlled on (could be monster or NPC though).