A slayer helmet is a black mask, combined with a facemask, nose peg, earmuffs and a spiky helmet.
A full slayer helmet is a black mask, combined with a hexcrest, focus sight, facemask, nosepeg, earmuffs and a spiky helmet. A full slayer helmet can be painted with loyalty points
A hexcrest can be worn in the helmet slot, it increases magic damage by 15% on slayer tasks.
A focus sight can also be worn in the helmet slot, it increases ranged attack swell as ranged damage by 15%.
A black mask is worn in the helmet slot, it provides a 15% attack boost and 15% strength on slayer tasks.
A (full) slayer helmet is roughly an equivalent to a rune full helm. However, it is also like a void set in one piece.
Hope this helped.
A black mask