dark
The German word 'dunkel' is used to describe something that is dark or not well-lit. It can be used as an adjective, such as 'ein dunkler Raum' (a dark room), or as an adverb, such as 'dunkel leuchten' (to glow dimly). 'Dunkel' can also be used metaphorically to describe something that is obscure or difficult to understand, such as 'dunkle Zeiten' (dark times).
The room is dark.
This sentence describes a state of a location, indicating that there is not much light in the room.
He has dark hair.
This sentence is used to describe a physical characteristic of a person, in this case the color of his hair.
The dark chocolate is bitter.
This sentence describes the taste of dark chocolate, which is usually more bitter than milk chocolate.