The Genestealers (Corporaptor hominis) are one bioform of the multispecies Tyranid race that was genetically designed by the Hive Mind for the infiltration of other intelligent species’ settled worlds. The Tyranids are an intelligent, nomadic alien race governed by a collective consciousness known as the Hive Mind and comprising many different genetically-engineered forms.

(Source: http://http://warhammer40k.wikia.com/)
More fluff