Psychic ability is the skill which enables a person to read another person.

It is often referred to as linking into a person's aura and from that being able to talk about their past, present and future.  This does not involve a connection with a person in spirit, but a knowledge of how to tap into a person's psyche.

Mediumship is the abilty to tune into the energy or aura of a person who has died.  As the person has left their physical body they have move on into what are often termed the spiritual realms.  The medium is the person who tunes into the energy of the deceased friend or relative to pass on evidence and messages from them.