Creatures in 5e is a general term covering all living entities, from NPCs to PCs, wolves to level 20 gnome wizards. Monsters, characters, and NPCs are all creatures.
Monsters are creatures with a stat block, such as wolves.
Characters on the other hand appears to only refer to PCs with a character sheet, such a level 20 gnome wizards.
We also have NPCs, which are otherwise monsters, but the DM is using them to interact with the party socially.
I see some evidence of this in various sections. For example the proficiency bonus section states "Characters have a proficiency bonus determined by level, as detailed in "Step-By-Step Characters." Monsters also have this bonus, which is incorporated in their stat blocks."
This quote from the difficulty class rules states:
If the total equals or exceeds the DC, the ability check is a success–the creature overcomes the challenge at hand. Otherwise, it’s a failure, which means the character or monster makes no progress toward the objective or makes progress combined with a setback determined by the DM.
Are these definitions correct?