Sybill Trelawney's first prophecy

Trelawney's prophecy or The Prophecy refers to the prophecy Sybill Trelawney made to Albus Dumbledore at the Hog's Head Pub in 1980. It referred to Harry Potter and Lord Voldemort.

The Prophecy
"The one with the power to vanquish the Dark Lord approaches...Born to those who have thrice defied him, born as the seventh month dies...and the Dark Lord will mark him as his equal, but he will have power the Dark Lord knows not...and either must die at the hand of the other for neither can live while the other survives...."

- Prophecy made to Albus Dumbledore by Sybill Trelawney

The prophecy above was made to Albus Dumbledore by Sybill Trelawney and foretold the coming of a boy who would have the power to vanquish Lord Voldemort. The prophecy was made while Dumbledore was giving an interview to Trelawney for the post of Divination Teacher at Hogwarts School of Witchcraft and Wizardry. Dumbledore, who was disappointed at the performance she gave during the interview, was about to leave when she went into a trance and made the prophecy. Severus Snape, who was working for Voldemort at the time, was caught eavesdropping on Trelawney and Dumbledore by the owner of the Hog's Head and was subsequently thrown out of the pub. Snape then returned to Voldemort to tell him what he had heard.

Voldemort's Reaction
As Snape was apprehended before the full prophecy could be made, what Voldemort heard was only part of the prophecy. Oddly enough, the prophecy referred to two people; Harry Potter and Neville Longbottom. Though the prophecy referred to both Potter and Longbottom, Voldemort decided that it was Harry whom which the prophecy was referring to. Although Voldemort believed that a pure-blooded wizard was the only wizard worth being, he chose the half-blooded Potter over the pure-blooded Longbottom, this may have been because Voldemort was half-blooded as well. Shocked by Voldemort's decision, Snape hastened to Dumbledore and explained that Voldemort had come to the conclusion that Harry Potter was his to-be vanquisher and had vowed to kill him. He also explained his long love for Harry's mother Lily and asked Dumbledore if he could protect them.

Death of the Potters
After months of searching, Voldemort finally discovered where Lily and James were and that they were using the Fidelius Charm to conceal themselves. With the help of Peter Pettigrew, James and Lily's long-time friend and secret keeper, Voldemort was able to go to Godric's Hollow where he proceeded to murder Lily and James and attempt to kill Harry. In the process of trying to kill Harry, his killing curse backfired on him and destroyed his body, leaving nothing but a soul behind. Harry was protected from Voldemort's killing curse because Lily had sacrificed herself to save him imbuing him with a charm that prevented Voldemort from harming him. Dumbledore furthered this magic by sending Harry to live with his aunt and uncle.

Records of the Prophecy
There is only one known record of the prophecy it was kept in the Hall of Prophecies in the Department of Mysteries at the Ministry of Magic until 1996 when it was destroyed in the Battle of the Department of Mysteries. Harry Potter and Albus Dumbledore are the only two people who know the full contents of the prophecy. Though Harry told Ron and Hermione of what the prophecy meant, he never told them word for word.

Difference In the film
In the film adaption of Harry Potter and the Order of the Phoenix, the Prophecy goes:

"The one with the power to vanquish the Dark Lord approaches...and the Dark Lord shall mark him as his equal but he shall have power the Dark Lord knows not...For neither can live while the other s-survives."