A null person is the polar opposite of a person who can use some form of special energy (Magical Energy or Psionic Energy). This is a person for which such energies do not exist, they cannot be directly impacted by such energy. While a rock being thrown by such energy, or a fireball created with such energy, would indeed impact a null, healing or hurting spells, mind reading, or any other use of energy that directly impacts the person will effect them.
Magical Null
A person who cannot be directly effected by magic. Such a person is still able to be impacted by psionic energy and, curiously, can still use devices or objects that are made to use magical energy. That is, assuming such energy is not directed back at them, they can pick up a device intended to produce, say a bolt of lightening. That would work just fine, but the same person picking up a device for healing ones self, would not be able to produce any effect. They may still be effected by psionic energy.
Psionic Null
Similar to a Magical Null, only for Psionic Energy specifically, and able to still be effected magical energy.
Total Null
Can't be effected by either magical energy or psionic energy.