Short answers, pulled from the story.
Thetis is a sea goddess and one of the fifty Nereids, daughters of the ancient sea god Nereus. She is best known as the mother of Achilles by the mortal king Peleus, but earlier traditions, including a hymn by the seventh-century BC Spartan poet Alcman, describe her as a demiurge who created the universe.
Both Zeus and Poseidon courted Thetis but withdrew because of a prophecy that her son would become greater than his father. Since Zeus had himself dethroned his own father to rule the gods, neither was willing to risk fathering a successor who might do the same to them.
Proteus advised Peleus to find Thetis asleep and bind her tightly. She shapeshifted into flame, water, a lioness, and a serpent, but Peleus held on until she consented. Their wedding was held on Mount Pelion outside the cave of the centaur Chiron, attended by the Olympian gods.
According to the Argonautica by Apollonius of Rhodes, Thetis burned away Achilles's mortality in fire each night and anointed him with ambrosia each day. When Peleus interrupted her by crying out, she dropped the child and fled to the sea. A later variant, recorded in Statius's unfinished Achilleid written between 94 and 95 AD, says she dipped him in the River Styx, but the heel by which she held him remained untouched.
Thetis intervened repeatedly on Achilles's behalf. She persuaded Zeus to turn the war against the Greeks until Achilles's honor was restored after Agamemnon took Briseis. She had Hephaestus forge new armor for Achilles after Patroclus died, preserved Patroclus's body with ambrosia and nectar, and finally convinced Achilles to return Hector's body for ransom when the gods demanded it.
Thetis was actively worshipped as a goddess in Laconia, where Pausanias recorded priestesses, a hidden wooden cult image called a xoanon, and a temple refounded after a Lacedaemonian war with Messenia. Herodotus also noted that the Persians sacrificed to a figure they called Thetis at Cape Sepias, though he believed they were venerating a different deity under a familiar name.