It really depends on the context strictly speaking a Faith Healer is someone who uses methods that are not scientificly proven to heal an illness. This could be a Healer (the term refers to someone who practices homeopathic or herbal medicine)using herbs, or a Shaman praying to his ancestors, however I beleive that what the pevious commentors are thinking about are thinking of is a Pastor or someone similar praying over the injured or ill asking their God to heal them.
Its an "alternative" form of healing, a person who uses religion and faith in order to heal a person. Completely bullsh!t if you ask me. You see people depending on these "healers" to heal their children all the time in the news, long story short, their children die and they get charged with child neglect and homocide.
It us someone who preys on ignorant people there has never been such a thing
A faith healer is someone who heals other using prayer.