Short answers, pulled from the story.
Circe transformed the nymph Scylla into a sea monster by pouring poison into the spring where she bathed. Homer describes this creature as land-based and more dragon-like than later versions suggest.
The distance between her lair and the whirlpool of Charybdis fits within an arrow's range. Sailors attempting to avoid the whirlpools pass dangerously close to the monster.
A red-figure bell-crater in the Louvre dated 450, 425 BC shows Scylla as a maiden with a kētos tail and dog heads sprouting from her body. This form was prevalent in ancient depictions though very different from Homer's description.
Book Three of Virgil's Aeneid associates the strait where Scylla dwells with the Strait of Messina between Calabria and Sicily. The coastal town of Scilla in Calabria takes its name from the mythological figure.
Homer names Crataeis as the mother but mentions no father while Apollodorus suggests the father was either Trienus or Phorcus. Other authors list Hecate as Scylla's mother in texts like the Hesiodic Megalai Ehoiai and Hyginus describes her as the offspring of Typhon and Echidna.