Ghosts (亡霊 bourei) are the souls of humans who have remained in the physical world. Although they are souls, they have body temperature and are fully corporeal.
Ghosts and Phantoms
Ghosts (亡霊 bourei) and phantoms (幽霊 yuurei) are more or less interchangeable in normal Japanese language, but are distinct entities in the Touhou series.
Phantoms are not limited to humans, but can come from any living or nonliving thing in which there dwells a particular kind of spirit (気 ki), and due to their shapeless forms it's impossible to determine what kind of being they originally were.
Ghosts are the after-forms of humans who died and become phantoms and have lingering attachments that tie them to the world too strongly. Since, other than humans, there are no living things that die in this manner, there are no ghosts of beings other than humans. From the strength of that attachment, they preserve the form that they had while they were alive, as well as their body temperature, and since they are even able to touch living humans, one might not be able to tell that they are departed spirits if one is an outsider. They are not capable of passing through solid objects like phantoms are.
Generally, a ghost's weakpoint is its own dead body. For the type of ghost who became a ghost from not realising they were dead, seeing proof in the form of their own dead bodies would cause them to revert to a normal phantom. While ghosts who already know that they are dead do not share this weakness, it is common for them to hide their bodies to prevent them from being given a memorial service and achieving nirvana. The likelihood of this occuring has gone down, however, now that more bodies are being cremated.
Since the driving force for a ghost's existence is its attachment, it is common for them to exert a negative influence on the living humans around them just by their presence, which can eventually lead to death. This effect is particularly strong against the source of their attachment (eg. someone against whom the ghost has a grudge), but can affect anyone.