A Voodoo Doll is a doll or effigy that is used in the practice of Voodoo or Sorcery. The idea is that causing harm to the doll will relate into harm of a certain targeted person.
The use of a voodoo doll can be in an act of revenge or malice. That you intentionally wish to cause someone harm. In fiction it is used to actually cause some kind of pain or discomfort. The more real life application have more of a therapeutic purpose.
Acting out one's rage or spite on an inanimate object is far more healthy they attacking the real person. Though these practices are normally kept private do to the dark implications that comes with the idea of a voodoo doll.
The Japanese version of the voodoo doll is far simpler. It is a simply crafted doll made of straw. It is often poked with needles at impaled using a hammer and nails.
Anime / Manga
The use of the voodoo doll is present in some manga and anime series that involve some level of magic. Sometimes used to enact revenge. Other times for a comedic effect.
In the manga/anime series Nagasarete Airantou, the shrine maidenMachi often uses a voodoo doll to torture and harass her younger sister, Ayane; and others who speak ill of her. Whatever she seems to do to the doll relates into what happens to the person she's attacking.
In the manga version of the story she tried to show off her skills of sorcery with the voodoo doll for Ikuto. She stuck a nail into it, but didn't realize it was working. The victim was her younger sister who was further away. Later, she was rather pleased to see it was working and presented it to her mother.
In the manga version of the Negima! story, the girl named Sayo Aizaka, who is a ghost, is trapped within the area of Mahora Academy. She is able to leave along the classes' journey to the Magical World through the magic of Evangeline by possessing a cute, small doll that shares Sayo's appearance. In this doll she can be seen by all, talk with everyone freely, fly, and leave the school grounds.
To Sayo's shock, inside the cute doll is actually a Japanese style voodoo doll with her name on it.
She spends most her her time in the doll with her best friend, the class reporter, Kazumi Asakura. Sayo will ride one of the Pactio Artifacts Kazumi uses for her spying missions and other information gathering.
Code Geass (Picture Drama)
Code Geass Picture Drama : 04
In the Picture Drama that was a special addition to the Fourth Code Geass DVD, Sayoko presented Nunnally with a gift. Much to Lelouch's terror, this present was revealed to be a Japanese style voodoo doll. These events take place some time after Episode 9.