Spirits

A spirit -- or ghost -- is a soul who continues to exist after their mortal bodies perish. Bodiless and lacking a physical form, they wander the earth for their own reasons.

Biology
A spirit's biology is purely metaphysical -- they are comprised entirely of emotions and memories of their past lives. The exact material of their forms is an intangible substance called ectoplasm, which makes it possible for spirits to be seen. When manifesting, spirits can manipulate the level of ectoplasm in their beings as to make themselves visible only to certain kinds of people. For example, a fully-manifested spirit can be spotted by virtually every living thing on the planet, paranormal or not. When partially-manifested, a spirit may be invisible to ordinary humans, but can be seen by paranormal beings and humans. Finally, when not manifesting at all, a spirit can wander unseen by all, and at that point, only a spirit medium, a shaman, or other similarly-trained individuals can catch sight of them.

Powers and Abilities
In general, a spirit is only as strong as the measure of its willpower; spirits who were strong-willed in life will have remarkable ease commanding magic forces, while meager spirits will find their abilities woefully out of their limits. Strong spirits may come to learn how to affect the physical world by way of telekinesis, using sheer will to command their surroundings as they please. Spirits may also possess mortal beings, but it is hardly a straightforward task; factors such as the victim's mental resistance and the spirit's own willpower can affect the chances of a successful possession. In the end, a spirit can either bend the world to its will, or it succumbs to the will of another.

Spirits retain knowledge of everything they learned from their old lives, including knowledge of magic arts. As spirits are intangible beings and are thus unable to interact with the physical world, a spirit trained in sorcery will have more success in battle than a spirit trained in swordsmanship or any sort of striking arts (the latter of which can be circumvented if a spirit is able to find someone to possess).

As an aside, all spirits can choose to assume an appearance matching any point in their mortal lives. They can look older or younger at will and manifest any combination of clothing that they wore before -- but beyond that, spirits can do little else concerning their looks, being unable to adapt a completely different face or body structure.

Death and The Other Side
There are many reasons why spirits reside in the physical world; to say farewell to a loved one, to avenge themselves if they had been murdered, or, weirdly so, they may find their new existence more preferable to their mortal life. But no matter their reason for staying, all spirits are instinctively aware of the Other Side, the final resting place of all souls, be they mortal or immortal. A spirit may take the journey to the Other Side at any time, but once they do, there is no going back. No one knows what lies in store for spirits there, but many believe it is a place resembling the biblical Purgatory where souls await judgment based on their deeds in life.

Spirits can die. They may be immortal, but not invincible, and there exist methods and spells that can inflict on them a second death. An eradicated spirit leaves nothing behind - no memories, traces, anything - and there is no hope for a resurrection or an afterlife for them. Once a spirit is gone, it is gone.

Types of Spirits
Ghosts

The terms "spirit" and "ghost" are interchangeable -- for all intents and purposes, they are both used to describe a intangible, but sentient being whose behavior does not threaten the living. Ghosts may find themselves drawn to their friends and loved ones, but they will mostly keep to themselves and mind their own business. There exist communities where ghosts are able to exist in harmony; such places are abandoned towns, decrepit castles, sprawling fields, or any region uninhabited by the living. The fabled 'ghost town' legends are often the result of untrained, ordinary mortals catching sight of these ghostly congregations and running away in fright, ironically unaware of the ghosts's civility and peacefulness.

Spectres

Any spirit can possess the body, but haunting the mind is something only a spectre can accomplish. Classified as spirits with an unusually high aptitude for mental possession, a spectre's motive may vary between causing their victims nightmarish torment or eternal companionship. Not surprisingly, many people do not fancy the idea of allowing a spectre to take up residence in one's mind if there is the slightest chance of the former happening. Those fortunate enough to encounter a friendly spectre, however, can appreciate having a lifelong friend in their head for the rest of their lives.

Phantoms

Violent and dangerous, spirits are classified as phantoms when they have been completely consumed by emotions pushing to unrepentant violence -- hatred for the living or sheer insanity, it matters little because phantoms can't be reasoned with. All they want to do is destroy. When priests speak of exorcisms, they are usually referring to spectres -- when supernatural hunters talk about eradicating spirits, then it's about phantoms.

Considering a spirit's power is proportional to the intensity of their willpower, a phantom is exceptionally deadly and may be capable of telekinesis, possession, elemental control, and any magic arts they mastered in life. Luckily, phantoms are also known for their inability to let go of their mortal life, and their volatile emotions often keep them tethered to a place of great significance to them. Thus, phantoms that go on a killing rampage are almost unheard of, but one can bet that there are cells of ghost-hunters and exorcists waiting to stop them in their tracks.