Excerpt From "The Medium's Book" Chapter XVI #187 and #188

Two Principle Orders  |  Varieties Common to All Types of Medianimity

Varieties Common To All Types of Medianimity

Sensitive Mediums
Persons who feel the presence of spirits via a peculiar impression, general or local, vague or undecided.  Most of these distinguish good or evil spirits by the nature of the impression that they cause.

Natural or Unconscious Mediums : Those who call forth the phenomena spontaneously, without any action of their will and, for the most, part unconsciously.

Voluntary Mediums : Those who have the power of calling forth the phenomena at their will, though they are ineffective if the spirits are unwilling to cooperate). 

 (Kardec,  "Medium's Book", Ch XVI #187)