A scratch is like a tiny mark on the wall. It might break a little bit of the paint (the top layer of your skin), but it doesn't go deep into the wall itself. It usually bleeds a little bit, but not much, and it heals pretty quickly without a mark.

Example

1. I got badly scratched by my cat on my leg.
2. The itch on my arm made me scratch like crazy.
3. Careful climbing the rocks, don't scratch your knees!