Probably both. If by a ghost you mean an apparition or something with a vague ghostly form then a lot of things, creatures and people watch us from the lower fourth dimension. Including nature spirits (Deamons) and many many other thought forms, some of them man made. This reality is thought responsive so for example, If a whole load of people here on 3d Earth think about something together and for long enough then it manifests in the 4d lowerastral plains which is right next door. It can then manifest here in 3d. People who claim to have seen some of these mythical creatures are actually witnessing a thought form created because so many people have thought about it down the centuries. Thoughts create as they say. Clairvoyants can sometimes see them and clairaudients can sometimes hear them. Many of us can as well but we don't acknowledge it. Some of the negative thoughts we hear are placed in our heads by dis-incarnate beings. Many of these so called crazed criminals who said they heard voices in their heads telling them to do this or that actually did hear voices in their heads. Regardless of what mainstream so called medical professionals tell you. The next dimension is at a right angle to this one. That's why, if you are in the right state of mind through meditation, or even stoned, you can sometimes see movement out of the corner of your eye when you know there is no one there. In a heightened or meditative state you can sometimes glimpse into the next density up. Call it 3.1d or 3.2d. The Catholic church are satanic and should be avoided.
As for ghosts in the traditional sense, ie Humans who have passed on, the ones people see are those who often didn't make it back to the 5th Dimension and are stuck in the higher 3rd, lower 4th dimension of this Planet. She (Mother Earth) has many frequencies and vibrations. We just happen to be (In school) in the dense and solid 3rd dimension. What people describe as Heaven (If you are from this planet) is just the 5th Dimensional version of Earth. They live lives just like we do except everything is brighter and stunningly so. Everything has a sparkling quality to it, the trees move, there are more colours and the physics of the place make no sense. You can actually think something into being in 5d which is why we have to learn to control our thoughts down here.
When you die and the real you (Your light body) separates from your organic body, a lot depends on your vibration at the time of death. If you are in a low vibration, let's say you killed yourself for example, then you are going to be stuck here (Earthbound) until the time you were actually meant to die. Those are the ghosts that many people see. Some people in this situation are angry, because no one can hear them and they cause trouble. People shouldn't be mad at them. Light them a tealight candle and leave them a glass of cool water. That energy you provide them might be the tiny push they need to get to where they need to get to. Pray for them if you wish. They are often in a confused and upset state. PSALM 23 is good. Yes I know the Bible is plagiarised but these words have been recited so often that they too have become embedded in Universal thought fabric. It works. The light beings in higher 5d will hear your words and come to their aid. Some (Ghosts) refuse to leave and stick around in the lower 4th dimension. Others who have committed serious crimes and have been rejected by their spiritually evolved Ancestors and family bloodline in 5d also get stuck here until they get the chance to reincarnate and pay back their Karmic debt.
The lower 4th dimension of this Planet, though is also accessible to Aliens with the correct tech, so not only do we have Human light bodies right next door to us, we have a whole load of higher density creatures of all descriptions including beings like the Jinn who have been here since the Planet was formed. So a Ghost could be any number of things. The important thing to remember is this. We all have family members assigned to us when we step down into this vibration so sometimes the ghostly apparition you see might just be Great Grandma come to see how you are doing. Your Family visit you all the time. So if people often see ghosts then maybe set up and ancestral shrine and leave them some energy. Fruit, water, and light. Tealites are safer until you get used to working with fire. You don't want to burn the house down! Your ancestors can help you a lot in your life. Tarot cards are a good way of communicating with them. Try going onto the Ancestor pick a card youtube video and see what message your ancestors want to tell you. It may be the start of something big. As long as we study our birth chart which tells us what we planned to do before our memory was wiped, and keep our vibration high enough, we will return to the 5th Dimension. Unless of course you are one of those stupid enough to have sold your light body to a Jinn/reptile for cash. Like some of these idiot satanists and celebrities. Then you are in serious, serious trouble. There are far darker lower vibrations and versions of this planet. That's where their negative vibration takes them when they depart their bodies. Those you see being tortured in the reptilian videos show what happens to people foolish enough to sign contracts with them. It means you have given them consent to do what they want with you. Unless you were sent here on a specific task. You can only go in this Universe where your vibration matches In this free will Universe.