In myth, Cassiopeia was the wife of Cepheus (Cepheus was a king of Ethiopia according to most sources). The royal pair had a lovely daughter who was named Andromeda.
Our story begins with a fateful mistake. One day, Cassiopeia proclaimed that she was so beautiful that her appearance put the Nereids to shame. The Nereids were enchanting sea nymphs, and they were naturally divinely radiant in both face and form. Not surprisingly, the Nereids were insulted by Cassiopeia's boast. They shared their displeasure with Poseidon, the Greek god of the sea. Poseidon decided that this expression of mortal vanity and hubris was too much to bear, and so he came up with a suitable punishment. The sea god therefore sent a monster - which is described sometimes as a giant serpent - to seek revenge.
Poseidon's sea monster destroyed the land. King Cepheus was horrified by the turn of events, and consulted an oracle to learn how to pacify the anger of the gods. He was told that the only way to stop the destruction and appease the monster was to offer his daughter Andromeda as a form of payment. Cepheus complied. Andromeda was then chained to a rock and left as an offering to the serpent. And the king's daughter would have been sea monster bait had it not been for the sudden (and fortuitous) appearance of the dashing hero Perseus. Indeed, Perseus eventually succeeded in rescuing Andromeda from a most unappealing fate.
However, the story of Cassiopeia does not quite end with the rescue of Andromeda. For legend has it that the vain Cassiopeia did ultimately achieve star status. She was transformed into the constellation that bears her name, and this group of stars reminds us forever of Cassiopeia's somewhat dubious contribution to Greek myth.
Note - some variations on the spelling of the name Cassiopeia are Cassiopea, Cassiope, and Kassiepeia.
Who's Who in Classical Mythology