Foremost, it is recommended you understand that a medium has the abilities of a psychic, but some psychics do not have the ability to become mediums. One of the abilities that mediums have but not achievable by psychics is the ability to communicate with the spirits present in the afterlife.
Another thing that is beyond the capability of a psychic is the functionality of a medium serving as the channel between the afterlife and the world of the living. On the contrary, a psychic has the ability to use various spiritual channels to foretell the future.
Take, for example, a psychic who has the ability to provide for you about love, relationships, money, or career. A medium does not have this ability to foresee and foretell about the future. A medium usually makes access to the spirit world to transmit its message; whereas, psychic deals with the presence and aura that could influence the person's future.