What is a medium?

A Medium is an intermediary, a channel between the invisible and the physical.

On spiritualism a medium is a person who possesses one or more psychic gifts, allowing the spirits or souls of deceased people to communicate with people on earth through him or her.

In alternative healings a medium can be an intermediary of healing power.

What is the difference between a psychic or medium?

Paragnostic, Clairvoyant, Soothsayer - they are all terms that actually mean the same thing and are only functionally different. 

The medium is able to receive and transmit information through the helper (angels) or through a person who has passed over. The medium often receives images through the "third eye" (comparable to film images, one of which is burned into the retina, as it were). The medium is a conduit who must translate and transmit the information received in the best possible way.