Ghosts are created by someone or something that leaves a strong impression on the world. While this normally stems from emotional or painful events, moments of brilliance can also create impressions. Ghosts aren't the souls of individuals who had died, but are mere impressions that left a strong enough impact on the world and as such the person who leaves behind a ghost can theoretically be alive at the same time.
Someone who utilizes ghosts and other spectral beings is called a Valkyrie or Valkalla.
Regular ghosts are mere impressions, as such they aren't truly sentient. They can only understand things in the context of the moment they died and resolving the issue that made up the impression will erase them entirely. Ghosts typically function best in enclosed spaces and those they have connections to, but they also function in places where few humans will bother them since they can't stand up to poking and prodding, like the wilderness.
A ghost who has become too unstable is called Spectre. They are too unreliable to use or bind in the same manner one would a normal ghost, possibly exploding if handled carelessly.
Likewise, a ghost that has taken in too many negative aspects of different individuals changes from being a ghost to a Wraith. These ghosts are different from their roots as mere impressions and can be hostile and capable of causing physical harm.
Ghosts are typically used because they aren't seen as reliable to work with and burn out if you draw on them for power but , as impressions, they can affect their surroundings depending on how they were formed. A ghost who suffered a debilitating injury will share that same pain with all those around it, one that died from being burned or frozen will give off extreme heat or cold. While these sensations aren't real, they feel real enough.
By binding ghosts into weapons or items, their powers can be used and directed. An example would be Blake's Frost Hatchet, where a ghost of a woman who died of hypothermia enables it to freeze things it cuts into or have an area effect when the ghost inside is released.
They can also be used as a power source, but this runs the risk of using them up and wiping out the impressions entirely.
Ghosts can also be warded against by a circle or banished with salt.