Moaning Myrtle is the ghost who haunts the girls bathtoom on the third floor.
Moaning Myrtle was a student at Hogwarts who was a target for bullies. One time after being picked on becuase of her glasses and ran away crying to the girls bathroom. She sat on the toilet "seat" and started to calm down. It was then she heard a voice speaking in a different language. Even though she couldn't understand the voice, she new it was male. So, being it a girls only bathroom and all, she went to tell the person to get lost basically. But, as soon as she opened the door and looked towards the sink, she died.
The cause of her death was this. As she looked towards the sink, she was actually looking towards the tunnel to the "Chamber of Secrets." At the mouth at the tunnel was the Basilisk. If anyone or anything comes within eye contact of the Basilisk it will be petrified and die.
So in the end, you have to feel quite sorry for Myrtle... Well not really lol!