Poltergeists can haunt locations or individuals. The individual is allowed a saving throw vs. spells, with a penalty of half the poltergeist's hit dice. Poltergeists have no control over the possessed individual, but do get to throw things at a wider variety of people. Unless they've had the bad luck to possess a hermit, in which case they'll leave as soon as they figure it out. Since they're kind of dumb, though, it may take a while.
Poltergeists are stupid.
Smart ghosts don't end up as poltergeists.