"You mean it's not a drawing or a painting?"
"Is that a knife you're holding?
"How are you doing that?"
"I remember doing that in school with crayons!"
"So you can't erase it, right?"
Scratchboards are basically like a sandwich: a thin layer of white clay sandwiched in between a stiff board on the bottom and a layer of black ink on top. Images are created by using something sharp to scratch off the black ink and reveal the white clay underneath.
The sharper the tool the more precision one can achieve. Surgical scalpels and tattoo needles can make very fine, hair-like lines, and sandpaper can achieve a softer fuzzier look. Different tools and different pressures can create an unlimited variety of textures, which help to achieve a greater level of detail.