Ghosts are... dead people. Any kind of people! ...but dead.
They can't use any of the gods' magic, and even those with innate magic tend to be unable to use it. They are for the most part confined to the realm of the dead.
Appearance-wise, they are almost always in monochrome, usually matching the colour of their eyes. If their eyes are multiple colours, it usually mixes into a single colour. Some powerful beings that have died are able to make themselves different colours, though. As with most undead, their eyes do not have pupils. Usually they tend to duller colours.
Ghosts can make themselves look like they did at any point prior to their death. With difficulty, they can even make themselves look older than they did before they died. By default they look as they did when they died, including any injuries sustained. But they can make their injuries disappear, and cannot be permanently wounded or destroyed (except by a god, but gods don't tend to bother with ghosts). Ghosts can also change the clothes they're wearing just by thinking about it. Pretty handy! They can also wear normal clothes... but that would require them first finding normal clothes, and there aren't really any in the afterlife.
Ghosts are the souls of the dead, separated from their mortal body. From death they are meant to be guided by the reaper to judgment, and the afterlife. But sometimes, very rarely, they escape and stay in the mortal realm.
The reaper isn't real fond of that, and tries to hunt down ghosts who have escaped the afterlife.
Most ghosts can't use magic, but those who had innate magic and were extremely powerful in life may retain some of their magic...
When ghosts are in the mortal world, they are mostly corporeal and can be seen and touched like any other mortal. But they can focus their forms and walk through walls or even turn partially invisible. However, this use of ghostly power usually draws the reaper quickly.