Ghosts are the sentient forms of living beings that have been dead.
Possession: All ghosts are capable of possessing humans and inanimate objects at will. This also extends to breaking said objects. Very few remedies exist to expel ghosts out of possession - those include: destroying the object/killing the human and the Funnel Spell.
Flight: All ghosts can levitate and teleport. They can also pass through walls.
Invisibility: All ghosts can control whether or not they are visible in presence to humans or other animals. Usually at first they do not have much control over the ability, but later they can control visibility at will.
Intangibility: Ghosts cannot physically interact with objects.
Immortality: Being dead, they can't die.