Your normal traditional ghost - when somebody dies they stay around as a disembodied intangible spirit. No supernatural powers. No bodily needs. They know all the things they knew in their lifetime, they have their normal senses, they can move around, and they can talk to people. But they can’t interact with the physical world.
How would this affect human society and the history of the world? My initial thoughts:
No mystery about the afterlife - it’s right there for everyone to see. No Heaven or Hell, you just stay here on Earth. Which means no fear of eternal punishment or promise of eternal reward to motivate the living.
Everybody that ever lived is still around - Julius Caesar, Attila the Hun, Shakespeare, Mohammed, Einstein, Hitler, Elvis - they’re all still here. Would society be literally ultra-conservative?
Ghosts outnumber people - would they demand equal rights? Or would it be the living asking for equal rights? Would there be two parallel governments?
Inheritance - Would the dead automatically lose their property by dying? Could dead people buy property after their death? Would Lincoln or JFK still be President after they were assassinated? Who’d be the King of England?
Crime and punishment - What would the law be like when all victims could testify? Would execution still be a viable punishment? Would there be any secrets when ghosts can go anywhere and see anything?
Economics - I’d guess there would be entire professions of living people providing services to ghosts based on their differing abilities and vice versa.
Obviously, these are only a handful off the top of my head of all the differences such a major change would make. I’d like to see what ideas other people think of along these lines.