The Thomisidae are a family of spiders, including about 175 genera and over 2,100 species. The common name crab spider is often linked to species in this family, but is also applied loosely to many other families of spiders. Many members of this family are also known as flower spiders or flower crab spiders.

Spiders in this family are called "crab spiders" due to their resemblance to crabs, the way such spiders hold their two front pairs of legs, and their ability to scuttle sideways or backwards. The Thomisidae are the family most generally referred to as "crab spiders", though some members of the Sparassidae are called "giant crab spiders", the Selenopidae are called "wall crab spiders", and various members of the Sicariidae are sometimes called "six-eyed crab spiders". Some unrelated orb-weaver spider species such as Gasteracantha cancriformis also are commonly called "crab spiders".

Source: Wikipedia, the free encyclopedia