A raven is a large, black bird that's a scavenger — meaning that it likes to feed on dead animals. If someone tells you that you have "raven tresses," they are commenting on your hair color, not your eating habits.
If you see a large group of ravens, you can call them an unkindness of ravens, or a conspiracy of ravens. Raven is also an adjective for anything that's the glossy black color of a raven's feathers: "His raven hair was held out of his eyes with a sweatband." Probably the most famous literary raven is the one in Edgar Allen Poe's spooky poem "The Raven," especially in the repeated line, "Quoth the Raven, 'Nevermore!'"