There are different types of Alien/UFO/USO encounters that are categorized by 'Kinds' as follows:

1st Kind- This is the sighting of unidentified objects. Most people who speak about this refer to Aerial objects but i would like to believe that the term also accounts for submerged objects (Unidentified Submerged Objects or USO) as well.

2nd Kind- the second kind of encounter refers to actual evidence of the effects of the first kind. These could include electrical and magnetic interference, animal mutilation (6th Kind as well), crop circles, unusual heating....These are just to name some of the common physical effects. Some people have claimed to suffer from memory loss after an encounter of the first kind. Some even end up with scars that are unexplained (6th Kind). Physical damage to the terrain is also in this category.

3rd Kind- This encounter MAY involve communication, at least unilaterally, with a being (extraterrestrial) from an 'unidentified object' (UFO or USO). However, communication is not a key part in this definition. By witnessing the animated extraterrestrial being form the unidentified object one would have experienced a close encounter of the third kind. This 'Kind' could even include seeing inside of the 'unidentified object'.

At this point i want to recognize Mr. Josef Allen Hynek who defined a scale of the close encounters:

The term close encounter was coined by American astronomer and ufologist Josef Allen Hynek (1910 - 1986) in his 1972 book The UFO Experience: A Scientific Inquiry. Hynek proposed three types of close encounter:

First Kind: Sighting of one or more UFOs at a distance of 500 feet lor less.
Second Kind: Sighting of a UFO with associated physical effects (e.g. heat, electrical interference, etc).
Third Kind: Sighting of an animated being (presumably an alien but not specifically defined as such).

http://www.paranormal-encyclopedia.com/u/ufo/close-encounters/

According to Hynek, six subtypes for the close encounters of the third kind are in his scale;

In addition to this, ufologist Ted Bloecher has suggested two additional sub-categories:

G: Abduction (the same as a close encounter of the 4th kind). 

H: An alien is injured, captured and/or killed.


There are other Kinds that have been identified as follows: 

4th Kind- This is an actual abduction- someone being taken against their will by an Alien being.
Click HERE for accounts of abduction

5th Kind- The central focus of this definition is BILATERAL communication between the extraterrestrial and the human. See HERE for more information on this. 

6th Kind- This is the close encounter of the deadly kind. Incidents involving UFO/USO that results in injury to human and animal life including death. As mentioned earlier, this involves cattle mutilation and strange scars on the skin of humans and animals.

7th Kind- The seventh kind of close encounter (CE) refers to hybridization. It is when the extraterrestrial being mates with a human. It also includes artificial reproduction using human and alien DNA. This could result in a 'Star Child' being borned. The star children usually have unusual abilities- think of mutants...these children could simply be extremely brilliant and not have 'powers' like those from X-men.