[james0246]

Agreed. Thread title has been updated for manga fans to fully appreciate a discussion concerning the S-Rank technique.

Here's the Data Book entry for the technique (translated by ShounenSuki):

Summoning: Impure World* Ressurection (口寄せ・穢土転生, Kuchiyose: Edo Tensei)
Ninjutsu, S-rank, Supplementary
User: Orochimaru

Using a living person as a medium**, it calls back a deceased person to this world.

[picture of Orochimaru putting a fuda*** inside the Shodai Hokage's head]
↑Within this body is a human sacrifice!?

[picture of the resurrected Shodai and Nidaime Hokage]
→The deceased has to obey the user of the technique...

A sinful summoning technique, resurrecting a deceased person back into this world. It uses a living human sacrifice as a vessel to hold the deceased in this world. The sacrifice the deceased descents in, is covered by a shroud of dust in the shape the deceased's body had in life. The technique itself, because it's a kinjutsu that's too inhuman, had it's existence concealed...

*Edo (穢土, "Dirty Soil") is what Japanese Buddhists call the world normal, unenlightened people live in. The current, living world. The world of the people who have not yet have escaped the polluting thoughts (greed, hatred, delusion, etc.) that result in suffering.
**The word used for medium is "yorishiro" (依り代). A yorishiro is an object capable of housing a Kami (神, the shinto concept of gods), allowing them to occupy physical space. Persons can also house Kami, but they are called "yorimashi" (依巫). The fact that the word yorishiro is used here shows how inhuman this technique is, treating the sacrifices as nothing more than objects.
***A fuda, or o-fuda (御札) is a kind of talisman or charm, made of paper. It is placed on walls, pillars, doors , and windows as a guard against evil spirits and such. They are often seen in manga and anime as a kind of mystical weapon. The exploding tags in Naruto are fuda, for example.