Will anyone explain to me what is the difference between a poltergeist and a ghost? Thanks

Poltergeists aren't transparent (explaning why Peeves can throw things at people), so they can't walk through doors and walls, like ghosts can.
And they tend to be a little cheeky - like Peeves.

a poltergeist is a trickster who has the powers of a ghost, while ghosts are souls who can't leave the world

Ghosts are the spirits of people who were once alive and decided to stay on in ghostly form instead of moving on to whatever afterlife. Poltergeists were never alive au people.

Also, poltergeists are always mischievous tricksters, while ghosts have the personality they had when they were alive.