Ghosts
Spirits who died without completing something.  They can be put to rest by completing whatever task they have left- finding their body, their murderer, or just making sure a sibling or friend is okay.
Spectres
Spirits who just can't find their way to the afterlife.
Poltergeists
Vengeful sprits. They're often people who died a particularly violent death, and while most other ghosts have the appearance of a human, they tend to have animalistic attributes. They're not really in their right minds; they lash out at anyone who comes onto their property.
Humans
Some humans are ghost hunters, always chasing after the next supernatural creature in hopes to finally catch one on camera. Others are completely normal. Still others have the ability to see and hear spirits, even when they want to be hidden.