Ghost means spirit.
There are evil or demonic spirits roaming all over the world, as well as there are angelic spirits.
If you want to know more about ghost or spirits, try reading the bible.
The bible speaks of fallen angels, also called demonic or evil spirits, who were dumped by God here on earth because their leader, Satan, ambitioned to be like God.
These demonic ghost are the same spirits responsible for things that we call elemental beings, ghosts, evil ghost or spirits and human spirits roaming around to do harm to others or vengeance.
These evil ghost are responsible for evil that men do. They are the ones making influence to man to do evil.
This may sound ridiculous but this is what is being taught in the bible.
There are other theory about ghost depending on the ideology, culture, intellectual orientations or religious orientation. But I suggest that you try reading the bible first.
If you want me to help you out about this topic, just feel free to contact me.