There are no set definitions or categories of ghosts.  As a starting point, here are some of the entities Ghost Trackers look for.

Crisis Apparition
A ghost that appears only a few times, to warn people about impending danger.  These ghosts usually take human form.  These are hard to investigate, because they are unpredictable. 

Recommended equipment.  Notebook, still camera, audio recorded – all are portable and easy to use with little advance notice

Ectoplasm
A very common kind of phenomenon, ectoplasm appears as a glowing, swirling mist.  It may have a vaguely human form, or be fairly shapeless.  It appears and disappears quite suddenly, but tends to remain in view for some time.  Ghost Trackers who have witnessed ectoplasm also report feeling a strong energy presence, a kind of electric tingling sensation.   

Recommended equipment.  EMF or Geiger counters, to measure electrical activity.

Orbs
The most common kind of phenomenon at a haunting.  They look like small balls of swirling light that either glow or are see-through.  They are a great for beginning trackers, since they can be found at almost any investigation.

Recommended equipment.  Still cameras are idea.  Make sure the lens is clean and the room is relatively free of dust and insects, to avoid taking pictures of false orb images.

Plasmoids and Streamers
Similar to orbs, only fast moving and usually appearing trailing a tail of light or energy.  These are harder to see with the naked eye and are usually only revealed in photographs.  Quite common at investigations, though.

Recommended equipment.  Still cameras are essential.  Make sure the lens is clean and the room is relatively free of dust and insects, to avoid taking pictures of false orb images.

Poltergeist
A troublesome spirit who likes to makes its presence felt.  Rarely seen, they will often move objects, make loud noises and touch people.  These can make for good investigations, because they usually go on for an extended period of time. 

Recommended equipment.  Video and audio recorders to capture sounds and images.  A great opportunity to record EVP (electronic voice phenomena).  Also, equipment that measures environmental changes (EMF, digital thermometer) is essential.

Residual Haunting
A kind of haunting where the ghost or apparition does not seem to notice the Ghost Tracker.  It’s as if they are replaying a scene from their past live, over and over again.  These are good sites to investigate, since they are fairly reliable. 

Recommended equipment. Video and audio recorders to capture sounds and images.  EMF or Geiger counters, to measure electrical activity.

Shadow Ghosts
Another prized catch for Ghost Trackers, this thin shadows hide amongst the other shadows of the night.  Ghost Trackers usually just catch a glimpse of a shadow ghost before it disappears.  While they are frequent guests during an investigation, they are very difficult to record.

Recommended equipment. Always a hard thing to photograph, recent Ghost Tracker Investigations have had some success using digital video cameras.

Spirits
As opposed to apparitions, which seem to have a fixed role or pattern, spirits have distinct personalities and seem to be free to do, more or less, as they please.   They tend to interact with Ghost Trackers and others, and leave evidence of their activities – everything from smells, to temperature changes, to Ghost Trackers feeling that they are being watched or touched.  The classic ghost, spirits provide a great opportunity for investigators because they tend to stay in one location and manifest on a regular basis.

Recommended equipment.  You will need your full range of equipment to investigate this kind of phenomenon.  Video and audio recorders to capture sounds and images.  EMF or Geiger counters, to measure electrical activity, and digital thermometers to record cold spots.  Also, a note pad is essential, to keep track of those feelings, sounds and smells you equipment can’t capture.

Ghost Light
Just like the name says, ghostly lights with no obvious point of origin.  Usually they are white or yellow, but can come in a range of colours, and may change colour during a single appearance.  They are relatively easy to investigate since often they appear over and over again in the same location.

Recommended equipment.   Still or video camera to capture images.  EMF or Geiger counters, to measure electrical activity.

Vortex
May be related to orbs, these are funnel shaped energy masses, with a distinctive tread mark shape in the centre.  They seem to come in limited colours – white or light blue – and most often go unnoticed until they show up in photographs or on video.  A rare and prized find for Ghost Trackers.