Wikipedia - They are also commonly called "flower spiders" because they are most often found on flowers, lying in ambush for prey. Crab spiders do not build webs to trap prey, but are active hunters much like the jumping spiders (Salticidae).
They are called crab spiders because of their first two pairs of legs, which are held out to the side giving them (with their flattened, angular bodies) a crab-like appearance. Also, like crabs, these spiders move sideways and backwards more easily than forwards.