The Iron is a type of vitamin. When used from the Bag on a Pokémon, adds 10 Defense EVs to the target Pokémon per use, until it has 100 Defense EVs. It also increases the Pokémon's friendship a little.
The Iron Ball is a type of held item. The Iron Ball halves the holder's Speed and makes it grounded. If held by a Flying-type Pokémon, it takes 1× damage from Ground-type moves, regardless of its other types. This does not apply during Inverse Battles or if the holder is grounded by any other effect.