They can be sights, sounds, tactile sensations, or even an uncomfortable feeling a presence. They are distinguished from ordinary hallucinations by the fact that all people are prone to these phenomena, and the person need not believe the brain ghost to represent reality, although some do.
"I was alone in my house when I felt the presence of a ghost."
"Ghosts don't exist; it must have been a brain ghost."
"I thought I heard a baby crying, but upon realizing that I was alone, I realized it was a brain ghost."