Do ghosts or evil spirits exist? Can they cause us harm at will?

Ghosts are a product of the human imagination. They simply do not exist.

God has given us some information about the jinn, a type of creature given the same measure of free will as human beings. They are required to believe in the divine message given to Prophet Muhammad (peace and blessings be upon him) as they were required to believe in the message given to Prophet Moses (peace be upon him). Like humans, most of them do not believe. Those of them who believe are good ones, and those who turn away from the faith are the evil ones, or devils. Their world is different from ours. They do not harm us as we do not harm them. We cannot see them, although they see us.
And God Almighty knows best.