I found a great site explaining different constellations:
Andromeda – The Princess
She was chained to a rock and condemned to be eaten by a monster after her mother, Cassiopeia, insulted Hera. Perseus saved and married her.
Aquarius – The Water Carrier
Aquarius is based on Ganymede, the son of the king of Troy. Zeus spotted him and was enamored with him and brought him to the heavens to serve as cupbearer to the gods.
Aries – The Ram
Aries represents the ram of the Golden Fleece, who was originally given to Nephele by Mercury.
Cancer – The Crab
Cancer was the crap sent by Hera to pinch at Heracles’ heels while he performed his second labor.
Capricorn – The Sea Goat
Capricorn was created when Bacchus was feeding next to the Nile. He jumped in and his top half became a goat while the bottom half became a fish.
Cassiopeia – The Queen
Cassiopeia was placed in the sky by Neptune, but to humiliate her he placed her so that she was upside down for half of every night.
Cepheus – The King of Ethiopia
Cepheus was placed in the sky as a reward for travelling to find the Golden Fleece, he was also the father of Andromeda.
Cetus – The Whale
No one really knows why there’s a whale in the sky.
Corona Borealis – The Northern Crown
Araidne’s crown was set into the sky by Bacchus when he found her abandoned by her love, Theseus.
Cygnus – The Swan
Cygnus was once a man but was so saddened by the death of his friend Phaethon that he was transformed into a swan.
Draco – The Dragon
Draco was the dragon killed by Cadmus in his quest to found his own city after being exiled.
Eridanus – The River
Eridanus is the river Phaethon perished in.
Gemini – The Twins
Gemini depicts the twin brothers Castor and Pollux, one who was mortal and one immortal. When the mortal Castor died, Pollux begged Zeus to allow him to share his immortality. Zeus placed them both in the heavens in order to reunite them.
Hydra – The Water Snake
The multi-headed monster killed by Hercules.
Leo – The Lion
Leo represents the Nemean Lion also killed by Hercules.
Libra – The Scales
Libra depicts the scales of Virgo, the goddess of justice.
Lyra – The Lyre
Lyra is the lyre played by Orpheus, given to him by his father Apollo. All of the living things as well as unconscious things were charmed by Orpheus’ playing.
Orion – The Hunter
Orion was placed in the stars after Athena accidentally shot him with an arrow after being tricked by her brother Apollo.
The hero and killer of the Gorgon Medusa.
Pisces – The Fish
The bottom half of Bacchus when he transformed.
Saggitarius – The Archer
Saggitarius represents the centaur Chiron, who was renowned for his gentleness. He was accidentally shot by Hercules and placed among the stars by Zeus.
Scorpius – The Scorpion
Scorpius represents the scorpion sent by Apollo to sting Orion.
Taurus – The Bull
Taurus represents the placid, snow-white bull-form taken on by Zeus when he fell in love with the princess Europa.
Ursa Major – The Great Bear
Callisto, a virgin maiden of Diana, was raped by an enamored Jupiter that took Diana’s form. As punishment, Diana turned her into a bear, but Jupiter saved her by setting her up in the stars.
Ursa Minor – The Lesser Bear
Callisto’s son, Arcas, was turned into a bear by Zeus when he attempted to kill his mother, unknowing that she birthed him. He was also set into the stars with Callisto.
Virgo – The Virgin
The goddess of justice whose scales, Libra, sit next to her in the sky.