Myrtle is a girl ghost who died 50 years prior to the events in the books, when the Chamber of Secrets was opened for the first time. The monster that killed her was a basilisk.
Moaning Myrtle's most important role is in Harry Potter and the Chamber of Secrets, where she helps Harry, Ron and Hermione to find out about the monster inside the Chamber of Secrets.
Myrtle also helps Harry with his second task in the Triwizard Tournament, in Harry Potter and the Goblet of Fire. Though Cedric Diggory had told Harry to take a bath while thinking about it, and provided him with the password to the prefect's bathroom; it was Myrtle who told Harry how to solve the clue, by opening it underwater.