'Rusalka- In Slavic mythology, a rusalka was a female ghost, water nymph, succubus or mermaid-like demon that dwelled in a lake. She was considered a being of the unclean force. The ghostly version is the soul of a young woman who had died in or near a lake and came to haunt that lake; this undead rusalka is not invariably malevolent, and will be allowed to die in peace if her death is avenged. In most versions, the rusalka is an unquiet dead being. According to Zelenin, people who die violently and before their time, such as young women who commit suicide because they have been jilted by their lovers, or unmarried women who are pregnant out of wedlock, must live out their designated time on earth as a spirit.
Rusalki can also come from unbaptized children, often those who were born out of wedlock and drowned by their mothers for that reason. Baby rusalki supposedly wander the forest begging to be baptized so that they can have peace. They are not necessarily innocent, however, and can attack a human foolish enough to approach them.'
Well, that's the defination Wikipedia gave for the most part and I think it explains pretty well.