What is what ?

For the purpose of not submerging our readers under unknown terms, this page has been created in order to define these said terms.

Yūrei

 Equivalent term for "ghost" in Japanese language and a more general term englobing many others such as onryō, ubume, goryō, funayūrei, zashiki-warashi, jibakurei...)

In Japanese folklore, when a person does not meet a peaceful death, their spirit remains on the world of the living, condemned to wander endlessly until what was wronged is not undone.

Yūrei are often victims of murders, suicides, accidents or unproper funeral rites. Strong emotions can also make their death unusual, such as hatred, grief, love, jealousy, envy, remorse...
The stronger the emotion was, the more dangerous and powerful the yūrei is.

They often roams the places they were frequenting during their lifetime. Most of them however wanders around the place where they died.

How they are decribed physically differs from a source to another. However, they are generally depicted as what the dead looked like when he died. If they were injured, they were seen with their wounds, it is however not always the case.
Yūrei can wear the clothes they had during their death or traditional ones - a white kimono - and therefore let their hair unpinned and messy. Their skin can be pale or lively.
Some parts of their body can be nonexistent, with their extremities fading away like smoke. Their appearance can be imaterial or material, it depends - again - on the source they were taken from.

The easiest way to get rid of a yūrei is to perform an exorcism by a priest or to help them undo what was wronged in order for the spirit to find peace and leave the living world.

Yūrei, by Matthew Meyer

 *notice the peculiar headwear, it is a hitaikakushi - "forehead cover" in Japanese - that is an unique trait to yūrei

 

Sources:

https://en.wikipedia.org/wiki/Y%C5%ABrei
http://matthewmeyer.net/blog/2012/10/28/a-yokai-a-day-yurei/
http://yokai.com/yuurei/

Onryō

Onryō are yūrei driven by vengeance, they were victim of an injustice in their lifetime and return from the death to wreak havoc.

Unlike a yūrei, an onryō is the result of a person dying with - or from - intense emotions, often hatred, grief or love. The stronger the emotion was, the more dangerous and powerful the onryō is.

They are one of the most feared and relentless forms of yūrei, as their destructive power is enough to cause death and, sometimes, natural disasters, due to the fact that they are Earth-bound spirits (jibakurei).

Their appearance is eerie, often a mutilated, decayed form of their former body, representing the evil nature of their grudge that leads to death. 

They are often delivering curses, famines and deseases with them, destroying whatever they touch, as they only exist through blind revenge.

Getting rid of them is almost impossible as they often returns after being exorcised or are just too powerful to be contained.

However, their curse can still be avoided as it does not spread without victims. Their activity area is and will still be limited to the place they are haunting, unless someone spread the curse to further places like a contagious disease.

They are one of the most prolific ghosts in popular culture, often sources of eerie urban legends (Hanako-san, Teke Teke, Kushisake-onna...) and also appearing as main antagonists in Japanese horror films (Ring, Ju On, Carved, Teketeke...).

Onryō, by Matthew Meyer

*Onryō show to their victims the gruesome way in which they died, often reproducing this same fashion to them.

 Sources:

https://en.wikipedia.org/wiki/Onry%C5%8D
http://yokai.com/onryou/
http://www.theparanormalguide.com/blog/onryo

More to come...