Short answers, pulled from the story.
Scylla is a man-eating sea monster from Greek mythology who lives on one side of a narrow channel of water, opposite the whirlpool monster Charybdis. She is first attested in Homer's Odyssey, where she snatches six of Odysseus's sailors from the deck of his ship and devours them alive. Later myths describe her as a beautiful nymph who was transformed into a monster through jealousy.
The most influential transformation story comes from Ovid's Metamorphoses, in which Circe pours a vial of poison into the sea pool where Scylla bathed after Glaucus rejected Circe in favor of Scylla. Another version, recorded by Tzetzes and Servius, has Poseidon's consort Amphitrite poison the spring where Scylla bathed out of jealousy over Poseidon's desire for her.
Ancient sources disagree. Homer, Ovid, and Apollodorus all name Crataeis as her mother, while her father is variously given as Triton, Phorkys, Poseidon, or Tyrrhenus depending on the author. Hyginus identified her parents as Typhon and Echidna, and the Hesiodic Megalai Ehoiai names Hecate and Phorbas.
Virgil's Aeneid identifies the strait as the Strait of Messina, between Calabria in southern Italy and Sicily. The coastal town of Scilla in Calabria takes its name from the mythological figure, and local tradition holds it was the home of the nymph.
The phrase means being forced to choose between two equally dangerous or undesirable outcomes, similar to the modern expression between a rock and a hard place. It derives from the geography of the myth: the two monsters occupied opposite sides of a channel so narrow that avoiding one meant sailing toward the other.
Notable paintings include Agostino Carracci's 1597 fresco in the Farnese Gallery, Bartholomaeus Spranger's 1587 canvas now in the Kunsthistorisches Museum in Vienna, and J. M. W. Turner's 1841 painting now at the Kimbell Art Museum. Peter Paul Rubens painted the moment of Scylla's transformation around 1636, and Eglon van der Neer depicted Circe poisoning the water in a 1695 work held in the Rijksmuseum.