Of The First Kind (CEI) - A UFO in close proximity (within approximately 500 feet) of the experiencer.

Of The Second Kind (CEII) - A UFO that leaves markings on the ground, causes burns or paralysis to humans, frightens animals, interferes with car engines or TV and radio reception.

Of The Third Kind (CEIII) - A CEI or CEII which have visible occupants.

There are also two more unofficial classes of Close Encounter:

Of The Fourth Kind (CEIV) - Abduction cases. 

Of The Fifth Kind (CEV) - Communication occurs between a person and an alien. Hynek Classification System

A system used to categorize the various types of UFO sightings invented by Dr J. Allen Hynek. The categories are as follows:

Nocturnal Light - Any anomalous light(s) seen in the night sky whose description rules out the possibilities of aircraft lights, stars, meteors, etc.

Daylight Disk - UFOs seen in the distant daytime sky. The UFOs classed in this category can be other shapes as well, like cigars, eggs and ovals.

Radar-Visuals - Where UFOs are tracked on radar and can be seen at the place illustrated at the same time.