What's the difference between a ghost and a spirit?

Ghosts
Some spirits retain memories and personalities of those long gone. They substantiate the popular belief in ghosts who became lost on their way to the Maker's side, even as it is against the Chantry teachings. Known examples include Katriel, Eleni Zinovia, Claudio Valisti, Cole, Telana and Justinia V among the others.

So: not every spirits are ghosts, but every ghosts are spirits.

Those two Avvar guys turned into stones in DAA did not seemed to be spirits to me,but they weren't alive either and they never meet a necromancer.... so other than ghosts how should I call them?

They were not spirits or ghosts. They were petrified humans. People who have been turned to stone and remain alive are a thing in DA. Remember Eleni Zinovia? The three pirates from Mark of the Assassin?

The three pirates could be returned to flesh, but their curse had an escape clause.

As for the Queen of the Blackmarsh ... could be a ghost, could be a spirit, could be the result of the Baroness' magic.

That's one possibility. Another is that the Sha-Brytol stole his body for some unknown reason.

As has been stated, the actual existence of ghosts, of lingering spiritual presence of a living being in the physical world, hasn't really been confirmed in Dragon Age. Most all we've encountered have indicated or left open the possibility that they are spirits, not unlike Cole to use an extreme example, mimicking the thoughts, memory and appearance of the dead.

That being said, we know people can continue to exist in the Fade after their body has expired, see Niall in DAO. This also seems to be what happened to the Baroness and her subjects in the Blackmarsh, assuming that really is the Baroness. If so, then it would seem mortal souls ultimately become more and more like spirits the longer they spend in the Fade.

EDIT: We also get lyrium ghosts, which mostly seem to just play out an event like we sometimes see in the Deep Roads. Those mostly seem to have been echoes preserved and given form by ambient lyrium than ghosts. Normal spirits do this too, like at Soldier's Peak. So spirits/lyrium ghosts seem to range between passive echoes and actively taking on the identity of and acting as a dead person, like Cole and Leliana.

I think that was referring to the spirit dragon.

I think Justice's words suggest she's a ghost rather than an abomination. Justice says "she assumed that role" when speaking of the Baroness we meet, which seems to me to suggest either that the woman we meet is a formerly-mortal Baroness who assumed the role of a demon, or that she is a demon who assumed the role of the Baroness. Furthermore, the quote "there was such a mortal once, but now? That is a demon of Pride" seems to suggest that the demon and the mortal mage who originally used that title aren't really the same person.

The same guy who has you prepare the Mortalitasi skull states that the dead can't return, and that necromancy just uses Fade spirits. Therefore he probably doesn't use or even know of any power that can recall the spirits of the dead. Therefore the skull probably doesn't.