Shadow ball is a special move. You can see special moves as moves that doens't make any pshycical contact like flamethrower, shadow ball, moonblast, dazzling gleam, energy ball etc...
Weaviles sp.attack is really low while it's attack stat is really high. pshycical moves are moves that makes, you know, psychical contact like ice punch, leaf blade, thunder pucnh, hammer arm and earthquake for some reason.
when you search for a move on this site you see an picture on the left that shows that it is a special or psychical move. special moves have a blue square with white cirkels in the middle while psychical moves have an orange square with a yellow esplosion thing in the middle. thats how I learnt what psychical and special moves were back when I started and it really helped me.
I hope I cleared things up for you and good luck on your journey