Ghosts are the spirits/souls of the dead who have failed to pass on to the afterlife. Not having the strength to make the transition, they instead cling to the Source like moths to a flame.
Ghosts usually take on the form and clothing they had at the time of their deaths. Occasionally a ghost will take on an idealized appearance, and/or a younger appearance from their "prime". On the other end, they can assume an exaggeratedly horrifying appearance, far worse than they looked even in death.
For unknown reasons, the presence of existing ghosts tends to make it more likely that those that die within their vicinity will become ghosts as well. Thus, because ghosts are tied to specific locations, means that quite often when you find one ghost, you'll find others. A "haunting" of ghosts, but to say they work in groups would be disingenuous as their unstable mental states make such group dynamics nigh-impossible.
Intangibility. Intangibility is actually their default state, they can through concentration and practice become tangible for brief periods, however.
Limited Shape-Shifting. Since they have no physical form, this isn't true shape shifting, but they do have some minor control over how they appear to others.
Limited Fear Control/Illusion.
Flashbacks. Ghosts are capable of manifesting a hyper-realistic vision of moments from their own life/past and drawing the living into it. This is more tangible than a mere illusion but is not true time-travel as much as it appears so.
Possession/Mind-link. Ghosts are capable of possessing living beings but this is extremely difficult for "young" ghosts and is usually a power associated with those who have been dead longer. This only works when they are aware they are, in fact, dead. To a lesser extent, they can also form a link with the minds of the living, not controlling them per se but able to share thoughts/dreams and induce a hypnotic/highly suggestive state.
Ghosts are already dead, meaning that they cannot be killed. They are immune to "human" weaknesses such as disease and aging.
A ghost’s consciousness is non-linear and resembles those of living humans who suffer from dementia and/or short-term memory loss. They need to concentrate to hold on to the present as well as in particular the knowledge that they are in fact dead. They are unable to hold on to it for too long before they forget once more.
This mental instability also leads to instability in their powers, causing them to be unable to hold on to physical form, etc.
Ghosts are all initially tied to a particular location, specifically where they died. There are ways for them to be free from this, but it takes a lot of effort and thus is very rare, as most ghosts are unable to hold on to lucidity long enough to accomplish such tasks. All ghosts are able to roam freely on Halloween/Samhain/All Saints Day, however.
They are also vulnerable to exorcisms and unable to fully manifest in daylight.