This is a single-replacement reaction that takes place according to the following equation:
`Fe_((s)) + NiSO_4_(aq) -> Ni-(s) + FeSO_4_(aq)`
Iron replaces nickel because it's more reactive that nickel. To determine if one element will replace another, consult an activity series table. An activity series lists metals in order of high reactivity to low reactivity. A metal that is higher on the activity series will replace one that's lower.
A single replacement reaction is also an oxidation-reduction, which means that electrons are transferred from one substance to another. In this case iron is oxidized from an oxidation state of 0 to +2, and nickel is reduced from an oxidation state of +2 to 0. Two electrons per atom were lost by iron and gained by nickel.