Yūrei:

They're a popular part of Japanese folklore, in the same vein as the Bloody Mary ghost, or the Annabelle doll. In their book "Living Folklore: An Introduction to the Study of People and Their Traditions", Sims and Stephen talk about how folklore exists in many places and is tied to the culture/history of that place(Sims and Stephen, pg. 2, 2011). This is very true in the case of Yūrei(pronounced as "you-ray"). They've been a part of Japanese culture since the early 17th century, and to this day, they still play a role in Japanese society(media, literature, traditions).


History of Yūrei:

According to Voyapon, death is almost as important as life in Japan. When a person dies, their soul travels to the yominokuni (黄泉の国), the Shinto afterlife, or the anoyo (あの世), the afterlife of the Buddhists. However, this road to the afterlife is not easy, and any obstacle towards that goal can make this spirit turn into a Yūrei . They are very important in the world of storytelling, legends, rumors, traditions, and mysteries in Japan to this day!

In Shinto tradition, it is said that everyone has God within them. When a person dies, God frees themselves from the body of the deceased. When a family member dies, the living relatives have to watch over the deceased, and help them during their journey to the afterlife by doing certain rituals. Once they have overcome all obstacles to the afterlife, this ancestor will watch over their living ancestors on the earth to protect them from any misfortune.  Unfortunately, those who have had an unnatural death,  left issues unresolved, or did not have a well-performed ceremony, could be stuck between life and death, creating Yūrei.

The roots of Yūrei can be traced from the Heian period (794 – 1185), when people created religious practices to appease spirits and the Gods. They even created the Goryo Shinko religion, where the temples were devoted to these difficult souls

 
Yūrei Appearance:

In general, the appearance of a Yūrei is fairly simple. They have the shape of a human, but no feet. They're usually wearing a white kimono(from their funeral), and may or may not have facial deformities. Of course, the most common is the long, white hair that covers the face! If you're ever watched Japanese horror movies or even some American horror movies about ghosts, you might be able to picture the appearance of the Yūrei.

Onryō:

The most well known yūrei is the Onryō. These are ghosts of people who died with extreme emotions such as jealousy, rage, or hatred—that their soul is unable to pass on to the afterlife. After their death, they transform into powerful, angry spirits who seek vengeance on everything and everyone they encounter. They might be victims of war, catastrophe, betrayal, murder, or suicide, and they display wounds or marks that indicate the way they died.
  

They only have one motive; vengeance. Onryō are powerful enough to kill anyone but they prefer letting the object of their hatred live a long life of torment and suffering, watching loved ones die in their stead. Onryō inflict a terrible curse on the people or places that they haunt. This curse can be transmitted to others like a contagious disease, creating a circle of death and destruction far more devastating than any ordinary ghost. Onryō just want to destroy, and they don't care about who they target. Their vengeance can never be satisfied. While most yūrei only haunt a person or place until they are exorcised, Onryō’s continue to infect a location long after the ghost itself has been laid to rest. Occasionally, an Onryō’s curse is born not out of hatred and retribution, but from an intense, passionate love that transforms into jealousy. These vengeful ghosts will haunt former lovers, evoking their wrath on any new and developing romances, second marriages, new children. They eventually end up destroying the lives of the ones they loved so much in life. No matter what the origin, Onryō’s wrath makes it one of the most feared supernatural entities in all of Japan.

Appearance of Onryō:

Onryō appear as slender females, dressed in white, with blood-stained clothes. They usually have green and purple veins and long, black hair covering their faces. When they become angry, their hair moves back, revealing a deformed face or  a face with a mouth, but no eyes nor nose, appearing almost faceless!

Onryō in Japanese Culture Today:
 

In current Japanese society, the Onryō is most well known for appearing in several Japanese horror movies, a concept that really took off in the early 2000s and was replicated in Western media. Specifically, I'm talking about the Ringu movie series and the Ju-On franchise(known as The Ring series and The Grudge series in the USA), and the vengeful ghosts of Sadako and Kayako, which are two prime examples of the Onryō. Both these figures, especially in the Ju-On(Grudge) series is inspired by a particular onryō legend. It is the myth of Oiwa, which is the tale of a young woman who was brutally disfigured and murdered by her horrible husband in an elaborate plot. The story of Oiwa appears in Yotsuya Kaidan, or The Ghost Story of Yotsuya. This particular story has been retold many times, in books, kabuki(Japanese puppetry), and film. There is a legend, that a curse accompanies her story, and that those who retell it suffer injuries and even death. To this day, producers, actors, and their crews continue to visit the grave of Oiwa in Tōkyō before productions or adaptations of Yotsuya Kaidan, praying for Oiwa's soul and asking for her blessing to retell her story.