Spectres are your run of the mill ghost. They haunt a place or object, and cannot leave that place except under very certain circumstances, usually to contact the last of a bloodline or a loved one.
How much of their sanity, personality, and intelligence has survived death differs from ghost to ghost, but they all have some things in common: They all have unfinished business that was supremely important to them, they are all incorporeal, and they are all bound to expressing themselves in metaphor. However, they may speak to other undead freely. Oftentimes, a spectre's 'unfinished business' is having his body located and properly buried. Spectres cannot directly affect living flesh, but they can move objects with concentrated effort. They tend to look as they did when they died, but enough have taken on appearances that differ that it is not a hard and fast rule.
Spectre is more of a catchall than a real category. If it's not an Angel, Ghoul, or Revenant, then it's filed away into Spectre, Subcategory Whatever. Poltergeists, O-Bakemono, Will-O-Wisp, anything not a part of the other three is a Spectre of some sort.
Dragons, look at them, etc.