The malevolent spirit called up by the Bloody Mary ritual is always said to be female, usually a female whose face was disfigured as a result of a violent death (could by car accident or murdered, dependents on which version.
In one version, Mary was a beautiful woman in life, who was very proud of her looks, to the point of self obsession, thus the spirit is summoned to appear in a mirror.
In most versions of the Bloody Mary legend, the evil woman could be summoned by chanting Bloody Mary into a mirror anywhere from three to hundred times in a darkened room lit only by a candle, 13 times seems to be the popular number chanted. The bathroom is the popular setting to test the legend but other rooms could be used. After the correct number of chants were given, the spirit would then appear in the mirror and claw the eyes out of the chanter and death would follow.
Other versions of the story have the spirit driving the chanter insane or pulling them into the mirror.
Who Bloody Mary is remains a mystery.
There is a version of the story that states Bloody Mary was Mary Worth, who was horribly disfigured in a car accident.
There is another version that Bloody Mary was a witch who was burned at the stake and returned for her revenge.