The 4 classes of spirits are as follows (note: some beings may fall under multiple categories):
Class 1: These beings are visible to human eyes, although they may be transparent. They are, however, intangible and general do not notice outside stimulus. Ghosts generally fall under this category.
Class 2: Commonly referred to as poltergeists, These beings can move physical objects, sometimes many at a time.
Class 3: Illusionairy beings that can manifest themselves only in dreams, hallucinations, etc. They generally work for a specific person or cause, but some (such as shades or nightmares) serve only chaos.
Class 4: Possibly the most dangerous of the 4 classes, Class 4 beings can possess living creatures, sometimes killing them afterword. These beings tend to be chaotic or evil in nature, and see themselves as above the other 3 classes. Bill Cipher, for example.