Spirit entities are many and varied.  The Spirit Realms change frequently, and therefore its denizens are mutable as well.   Scholars of Spirit Magic have identified five major categories of spirits, however, into which they try to cram any spirit they encounter.  Usually, this system works.   Sometimes it doesn't, and can result in confusion, miscomprehension, and peril.
The five categories are:  Naturae, Elementae, Entitae, Corruptae, and Mortitae.
Each of the five categories is discussed in its own section.  In addition, sample spirit types are listed within each section, with their relevant characteristics for Spiritism.   Spirits within each "type" vary greatly, however, both in terms of specific behavior interaction with adepts and psychics, and in their potential capability for Spiritism.
bullet.gif (892 bytes) Naturae are the spirits which most closely relate to animals and plants.  Throughout the Spirit Realms that are "nearest" to material Geoza, these types of spirits dominate.  All higher-order animals and plants seem to be represented by spirits, who in turn possess most or all of the behavioral nature of their material counterparts.  Thus, a Wolf Spirit will behave as much like a wolf as is possible in the Spirit Realms.  An Oak Spirit will exhibit the qualities of an oak tree which are relevant to the Spirit Realms.
The Spirit Realms seem to mirror the material world in this regard, at least in those areas "closest" to the material world.  A person who enters the Spirit Realms from a forested hillside will not appear in a spirit desert, but in a realm which closely resembles his material entrance location.  There will be spirit "trees" and other Naturae which are symbolic of the material environment.
bullet.gif (892 bytes) Elementae are the spirits of the natural forces in Geoza: rain, wind, rivers, fire, and earth, to list just a few examples. 
Like Naturae, Elementae represent those aspects of the material world which are "closest" to the Spirit Realms.   One would rarely find a River Spirit soon after entering the Spirit Realms in the middle of a desert, for example.  An adept or psychic entering the Spirit Realms from a ship at sea would tend to encounter spirits of Wind, Storm, or Sea, not Mountain Spirits.
bullet.gif (892 bytes) Entitae are those spirits which seem to exist not in symbolic reflection of material existence, but on their own merit as spirits.  A Wolf Spirit, for example, has no identity other than its reflection of the material animal, the wolf.  There may be many wolf spirits, but none have any true individual identity.
Entitae, on the other hand, are all individuals.  Some may share characteristics or capabilities (and thus can be grouped together as a "type" of spirit), but only an inexperienced or foolish adept forgets that each spirit in this category is an individual being.  Thus, interactions with Entitae are much more complex (and often more difficult for an adept or psychic).
bullet.gif (892 bytes) Corruptae are those spirits which represent the corruption of the Unlife.  Even in the Spirit Realms, the Unlife has made incursions, and some areas are nearly completely devoid of any spirits but Corruptae.   Such areas are wisely avoided by adepts and psychics.  Corruptae may have at one time been almost any other type of spirit.  For some reason each was corrupted by the Unlife, and now reflects some aspect of Unlife existence, rather than the Geozan material world.
Naturae are the most commonly corrupted, followed by Elementae.  Entitae seem to be more resistant to corruption.  This may be due to their individuality.  Those Entitae which have been corrupted are true terrors, however.  Their individuality has been subsumed into the Unlife, and they are both more powerful and more willful as a result.  Such spirits may attempt to masquerade as uncorrupted Entitae, and seek out inexperienced or careless spiritists.  Such adepts get far more than they bargained for, and few live to regret it.
bullet.gif (892 bytes) Mortitae are the spirits of people (Humans, Dwarves, Kri'ik, Hssuga, or Aezjareans) who have died.  Not all spirits of the dead remain in the spirit realms, but some do.  The spirits of the recently dead are often very interested in material reality, especially in those things that interested them when they were alive.  The spirits of the ancient dead are often actively disinterested in material reality, preferring instead the spirit version of life that they have wrapped around themselves over the ages.
Mortitae provide a haphazard capability to spiritists who call on them.  Their ability to participate in Spiritism is even more erratic than for other spirits, and they are sometimes oblivious to their potential powers.  Those who have some intense spiritual focus (avenging a wrong, a favorite haunt, etc) are often capable of powerful mystical acts, sometimes even without the aid of a spiritist.  A wise spiritist will learn as much as possible about the person she wishes to summon before she does so.  It is often the secrets that people hide from each other that have the most enduring existence in the spirit realms.