Rock Head prevents recoil damage from its own attacks except for Struggle. It does not protect against damage caused by held items like Life Orb, Jaboca Berry or Rowap Berry. Crash damage from missing an attack is also not prevented.
If Rock Head is lost in a turn due to using a recoil-inducing contact move on a Pokémon with Mummy, the HP loss from recoil will occur that turn.
Generation III - Prevents recoil damage.
Generation IV - VI - Protects the Pokémon from recoil damage.
These are the moves learned by those with Rock Head that cause recoil damage. Other moves are affected but, are very unlikely to be on a Pokémon with this ability.
Pokémon with Rock Head