Creature is basically every living, breathing (or undead or constructs) 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 affects everything it targets.
Character is ambiguous by default and is used by itself to refer to both non-player characters and player characters depending on context. When it is ambiguous to which it refers, it should be clarified to be player or non-player.
Player character refers to characters controlled by players and is a subset of both humanoid and creature.
Non-player character (NPC) is usually a humanoid, but can sometimes refer to all DM controlled creatures. It is exclusive of PC and is a subset of creature.
Monster is exclusive of NPC and PC usually, but is still a subset of creature.
Humanoid is a subset of creature, and is generally inclusive of PCs and NPCs (though not always). It refers to anything even vaguely humanoid. Relevant quote:
The most common humanoid races are the ones most suitable as player characters: humans, dwarves, elves, and halflings. Almost as numerous but far more savage and brutal, and almost uniformly evil, are the races of goblinoids (goblins, hobgoblins, and bugbears), orcs, gnolls, lizardfolk, and kobolds. (BD&D DM book 2)
So in summary, when it says "creature", it means everyone, if it 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 one (could be monster or NPC though).