There are many types of medium but the one most commonly referenced are psychic mediums. Mediums claim to be able to facilitate communication with spirits of the dead, angels, demons or other immaterial entities, and share these experiences with non-mediums. The communication may occur in various forms such as direct speech, channelling, automated writing, Ouija boards and others.
People who seek out mediums often do so because they are in an emotional state, such as following the loss of a loved one. A medium will typically attempt to contact the spirit of the dead person. People who visit mediums often feel they reached contact with the lost one. But mediums will use a variety of entirely natural techniques to evoke this belief, such as the technique of cold reading which is composed of non-specific, generalized statements that are often vague enough that they might apply to a variety of people in several different circumstances.
Many Mediums claim they will be able to help their clients reach closure in their grieving process. Sadly the opposite is more often true. The client often becomes dependent on the Medium, a fact that many Mediums use to their advantage while they continue to charge for each session.
Beyond anecdotal reports and impressions, there is no hard evidence to support the claims of Mediums. It is one of the cruelest types of charlatanism, taking advantage of people in extremely vulnerable states, and giving them the false hope of communicating with loved ones beyond the grave. It often produces a prolonged grieving period and in many cases even prevents closure.