Something with a thin edge or a fine point that can cut is sharp. It's painful when your dog climbs into your lap, poking you with his sharp toenails.
A knife and a pointed stick are sharp, since they can pierce or cut, and a piercing pain in your neck is also sharp. You can also use sharp to describe a smart person or clever decision — or someone who dresses well, or the clothes such a person wears. Sharp also means clear, like sharp images in a book of black and white photography, or sudden, like a sharp turn in a mountain road.