According to Greek mythology, while Hercules was fighting the many-headed Hydra, a crab was sent to annoy Hercules and the crab bit him on the foot! Hercules stamped on the crab crushing it. The Goddess Hera then placed the crab among the stars.
In the sky at night the constellation looks like a upside down Y. You can see it in the Northern Hemisphere in Spring and the Southern Hemisphere in the Autumn.