It is challenging enough for the casual observer to tell the difference between the Fade and the creatures that live within it, let alone between one type of spirit and another. In truth, there is little that distinguishes them, even for the most astute mages. Since spirits are not physical entities and are therefore not restricted to recognizable forms (or even having a form at all), one can never tell for certain what is alive and what is merely part of the scenery. (It is therefore advisable for the inexperienced researcher to greet all objects he encounters.)
Typically, we misuse the term "spirit" to refer only to the benign, or at least less malevolent, creatures of the Fade, but in truth, all the denizens of the realm beyond the Veil are spirits. As the Chant of Light notes, everything within the Fade is a mimicry of our world. (A poor imitation, for the spirits do not remotely understand what they are copying. It is no surprise that much of the Fade appears like a manuscript translated from Tevinter into Orlesian and back again by drunken initiates.)
In general, spirits are not complex. Or, rather, they are not complex as we understand such things. Each one seizes upon a single facet of human experience: Rage, hunger, compassion, hope, etc. This one idea becomes their identity. We classify as demons those spirits who identify themselves with darker human emotions and ideas.
The most common and weakest form of demon one encounters in the Fade is the rage demon. They are much like perpetually boiling kettles, for they exist only to vent hatred, but rarely have an object to hate. Somewhat above these are the hunger demons, who do little but eat or attempt to eat everything they encounter, including other demons (this is rarely successful). Then there are the sloth demons. These are the first intelligent creatures one typically finds in the Fade. They are dangerous only on those rare occasions that they can be induced to get up and do harm. Desire demons are more clever, and far more powerful, using all forms of bribery to induce mortals into their realms: Wealth, love, vengeance, whatever lies closest to your heart. The most powerful demons yet encountered are the pride demons, perhaps because they, among all their kind, most resemble men.
--From Beyond the Veil: Spirits and Demons, by Enchanter Mirdromel.