A Poltergeist (騒霊, sōrei) is a being very similar to a ghost, and is often seen announcing its presence by introducing disorder. They are able to remotely manipulate objects. They can also manifest themselves visibly, audibly and solidly to the material world (it is assumed that not all ghost-like beings have such an ability).
Poltergeists seem to have a talent for discovering and causing the kind of interference that will raise the greatest commotion in any sane situation.
Speculatively speaking, their talent with interference potential may be what allowed the Prismriver sisters to become famous as the Phantom Ensemble. They make music that strikes all the right nerves in the audience.
Although ghosts and poltergeists are usually thought of as same species, in actuality they are not, according to ZUN. Likewise, poltergeists aren't phantoms, but are similar enough to be categorized with them.
(This is probably because poltergeists in Touhou (the Prismriver sisters in particular) aren't spirits of the dead, but constructs based on Layla's power.)