Thetis and Peleus were a famous couple (for several reasons) in Greek mythology.
Thetis was a Nereid, one of the daughters of Nereus and Doris, while Peleus was a mortal, son of king Aeacus and of Endeis.
Thetis was raised by Hera and she was very grateful to the Zeus's wife. That's why, when Hephaistos was thrown out of Olympus, Thetis took care of him, on the island of Lemnos. Also, when Zeus wanted to make her his lover, Thetis refused, because she wouldn't want to be unfair to Hera.
Another story, though, narrates that Zeus and Poseidon were both in love with her. But an oracle (or Prometheus) told them that the son born from the goddess Thetis will be stronger than his father. Zeus didn't want a son who might become the ruler of all gods, and of course neither did Poseidon want a son stronger than him. So they decided to marry Thetis to a mortal, Peleus (who wooed her in a very untraditional way, so I guess the marriage was meant as a punishment for her).
The wise centaur Cheiron advised Peleus to catch her by surprise, when she was asleep, and to also take a rope and tie her tightly. Being a marine divinity, Thetis was a shape-shifter. When Peleus tied her, she turned into fire, water, wind, a tree, a bird, a tiger, a lioness and a serpent. But Peleus held her tight and finally she turned again into a goddess and a woman.
Being defeated, she had to accept the marriage with Peleus. The wedding of Thetis and Peleus took part on Mount Pelion. They have received many gifts, among them there were the two immortal horses, Balius and Xanthus, which were later to be their son Achilles' horses.
All the gods were invited at the wedding... all, except Eris, the goddess of discord. So she decided to get revenge, by throwing among the guests a golden apple, with the words "to the fairest". Hera, Athena and Aphrodite decided, each one, that the apple was meant for her. Zeus didn't have the courage to get involved into this dispute, so he sent them to Paris, a prince who at that time was a cattle keeper.
And this is how the
wedding of Thetis and Peleus led to the Judgement of Paris, who chose goddess Aphrodite (after she had promised to give him Helen, the most beautiful woman on Earth) and thus caused the Trojan war.
So, as you can see, Thetis and Peleus were famous for their "wedding of the century" or "wedding of the millenium", because all the gods took part (except Eris, of course), and Peleus had the occasion to hear the muses sing. But all in all it didn't end well, because of the chain of events which started at the wedding.
Click here to read the rest of the story, about Thetis and Peleus' son Achilles and how his mother tried to protect him.
Click here to see pictures of the nereid Thetis.