Cancer is best noted among stargazers as the home of Praesepe (Messier 44), an open cluster also called the Beehive Cluster or the Gate of Men. The smaller, denser open cluster Messier 67 can also be found here.
The constellation of Cancer is a difficult one to recognize even when you are looking right at it, so imagine how hard it is to find if you do not know anything about it. While this star grouping represented a giant crab to the ancient civilizations that named it, it looks nothing like a crustacean, resembling an upside-down "Y" if anything.
Image credits: Cancer Till Credner. M44 NOAO/AURA/NSF