Sybill Trelawney is a character in the Harry Potter Universe. She is the Divination professor, however from 1996 onwards she shares this position with the centaur Firenze. She was the one who produced the prophecy that led Lord Voldemort to try and murder Harry Potter.
Sybill Trelawney most likely attended Hogwarts, though it is unknown which House she was sorted into. It is quite possible that she was a member of Horace Slughorn's Slug Club, since she was related to famous Seer, Cassandra Trelawney. It is also possible that Slughorn found her unfit for the Slug Club, but it is unknown how she behaved in her youth.
In 1980, she made an appointment to meet Albus Dumbledore at the Hog's Head. There, she would apply for the Divination professor position, which Dumbledore deemed unnecessary. He was about to politely decline Sybill's application when she went into a trance and made a prophecy about Lord Voldemort and an unnamed boy. Severus Snape was eavesdropping and brought the news to Lord Voldemort. Snape, however, had only heard half of the prophecy and did not know which boy the prophecy regarded. Lord Voldemort assumed the prophecy was about Harry Potter and set out to kill him. Fearing for Sybill's safety, Dumbledore decided to hire her.
 Harry Potter and the Prisoner of Azkaban
Trelawney is first introduced in Harry Potter's third year when he and his classmates begin to take Divination lessons. Most of them quickly begin to dislike the class, especially Hermione Granger, who quits the whole subject shortly afterward. She constantly predicts Harry's death to his growing annoyance.
Near the end of the year, she enters another trance, this time with Harry present in the room. She predicts that the servant of the Dark Lord will return to service that day, which turns out to be correct when Peter Pettigrew rejoins Lord Voldemort later that night.
 Harry Potter and the Goblet of Fire
During Harry's fourth year at Hogwarts, the Triwizard Tournament was held with Harry being one of the Champions causing Professor Trelawney to continue to shower him with death predictions. It is presumable that she predicted the death of Cedric Diggory at a time when Harry was not present.