der Bart
beard
All the hair that grows on men's faces, especially on the chin, cheeks, and upper lip.
Er trägt einen Vollbart, den er regelmäßig pflegt.
He has a full beard that he takes care of regularly.

beard
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beard
All the hair that grows on men's faces, especially on the chin, cheeks, and upper lip.
Er trägt einen Vollbart, den er regelmäßig pflegt.
He has a full beard that he takes care of regularly.

The main meaning is facial hair. The other meanings are metaphorical and become clear from context. If a 'Schlüssel' (key) or 'Schloss' (lock) is mentioned, the technical part is meant. With 'Pflanzen' (plants) or 'Tieren' (animals), the bristly appendages are meant.
Der Schlüsseldienst musste das Schloss austauschen, weil der Bart des alten Schlüssels abgebrochen war.
The locksmith had to replace the lock because the bit of the old key had broken off.
In connection with facial hair, the most common verbs are 'einen Bart tragen' (to wear/have a beard), 'sich einen Bart wachsen lassen' (to grow a beard), and 'sich den Bart rasieren' (to shave one's beard) or 'den Bart stutzen' (to trim the beard).
Er hat sich entschieden, sich einen Bart wachsen zu lassen.
He has decided to grow a beard.
'Bart' (beard) is the base word for many different types of beards. Examples are 'der Vollbart' (full beard), 'der Spitzbart' (goatee/pointed beard), 'der Kinnbart' (chin beard), or 'der Dreitagebart' (three-day stubble).
Für die Rolle im Film musste er sich einen Vollbart zulegen.
For the role in the film, he had to grow a full beard.
mustache / whiskers
'der Bart' and 'der Schnurrbart' both have to do with hair on the face or around the snout. For people, 'der Bart' is the more general term: it can mean hair on the chin, on the cheeks, and above the upper lip. 'Der Schnurrbart' means only the hair above the upper lip. So a mustache can be part of a beard, but not every beard is a mustache. For animals, 'Schnurrbart' often means the long sensory hairs, especially on cats. 'Bart' can also mean threadlike appendages on animals or plants, but then it sounds more technical.

Nach dem Urlaub hatte Max einen kurzen Bart am Kinn und an den Wangen.
After his vacation, Max had a short beard on his chin and cheeks.
Sein Schnurrbart ist schmal und liegt direkt über der Oberlippe.
His mustache is narrow and sits directly above his upper lip.