few types of ghosts - first type of it will be looks like a souls of villagers and pets. this ghosts will not be hostile(but it will be ready to kick your ass as far as you shatter peace and quiet of it), also it will cast some potion effects on you at interaction. This mobs will have an ability to interact with some objects - destroy torches, open doors, activate/disactivate levers... Another type of ghost will be hostile. It will cast more harmfull potion effects on you at interaction, but this ghost will not interact with any objects. All ghosts will have ability to go through the walls, and becomes invisible.