According to most traditions, the rusalki were fish-women, who lived at the bottom of rivers. In the middle of the night, they would walk out to the bank and dance in meadows. If they saw handsome men, they would fascinate them with songs and dancing, mesmerise them, then lead the person away to the river floor, to live with them. The stories about rusalki have parallels with the Germanic Nix and the Irish banshee. See Slavic fairies for similar creatures.
(taken from Wikipedia)