The Killing Curse, one of the three Unforgivable Curses from the Harry Potter series. Anybody caught performing or attempting an Unforgivable Curse will be sent to Azkaban Prison for life. The result of being struck by Avada Kedavra is instant death for no biological reason. Harry is the only person who's ever survived the spell, escaping with merely a lighting bolt-shaped scar on his forehead. Harry's mother Lily Potter died defending him from Lord Voldemort, and established a protection spell powered by the love for her child, and caused the curse to rebound upon Voldemort, leaving the scar behind on Harry. This love spell continues to protect Harry so long as he remains at Number 4 Privet Drive under his Aunt's care (because she is the only other blood relative of Lily who's still alive besides Harry) until he reaches adulthood (17). The protection keeps Voldemort from entering the house, and disables him from touching Harry without feeling excruciating pain. The skin protection is diminished when Voldemort is revived with Harry's Blood in Harry Potter and the Goblet of Fire, but the house barrier still keeps Voldemort out afterwards.
Voldemort's body was destroyed by his own wand's Avada Kedavra when he attempted to kill Harry the first time. He remained alive in spirit alone because his soul was split into at least 4 isolated objects hidden in various places significant to Voldemort. These objects containing pieces of Voldemort's soul are called Horcruxes. In order for Voldemort to be truely killed, his Horcruxes must be destroyed before his body.
Harry survived Avada Kedavra again when he stopped the Avada Kedavra with his Expelliarmus (Disarming Charm), and initiated Priori Incantatem (a sequence of events that occurs when two wands with the same magical source are forced to duel eachother. Harry's and Voldemort's wands shared feathers from the same phoenix, Fawkes. The wands are referred to as "brothers").
The only other known way to defend yourself from Avada Kedavra (besides the protection of love and wands sharing a core) is to escape from it somehow, either by dodging it or hiding behind a solid object.
A swishing noise and a second voice, which screeched the words to the night: "Avada Kedavra!"
A blast of green light blazed through Harry's eyelids, and he heard something heavy fall to the ground beside him.
Cedric was lying spread-eagled on the ground beside him. He was dead.
For a second that contained an eternity, Harry stared into Cedric's face, at his open grey eyes, blank and expressionless as the windows of a deserted house, at his half-open mouth, which looked slightly surprised.