And how much damage do these items inflict?
There are actually 3.
Rowap Berry:
When a Pokémon holds a Rowap Berry and is hit by a special attack, the attacker will lose 1/8 of its max HP.
Jaboca Berry:
When a Pokémon holds a Jaboca Berry and is hit by a physical attack, the attacker will lose 1/8 of its max HP.
Rocky Helmet:
If a Pokémon uses a move that makes contact with a Pokémon holding a Rocky Helmet, the Pokémon that used the move takes damage equal to 1/6 of its maximum HP.
The Sticky Barb damages the user:
At the end of the turn, the Sticky Barb damages the holder by 12.5% of its maximum HP each turn. If a Pokémon with no held item hits a Pokémon holding a Sticky Barb with a contact move, the Sticky Barb will transfer to the Pokémon that used the move. The Sticky Barb does not damage Pokémon with the Ability Magic Guard.
Source: Bulbapedia