From the publisher blurb
Genestealer Cults are the insidious result of a Genestealer or its spawn entering a large mortal population and starting to procreate, either through infection or plain sexual intercourse. Through this reproduction more Genestealer Hybrids come into existence, and this creates a Hive Mind, a mental link which connects members of the brood and allows them to communicate and coordinate their actions. The more members a certain brood has the more effective it becomes and eventually creatures such as Patriarchs and Cult Magi come into existence, and a true Cult is born; from there they can start infiltrating and converting the human population.
If heretical cults weren’t bad enough for the Imperium to cope with, once matured these cults can truly destroy a community over time, or worse attract a Tyranid hive fleet.
The first generation is of such a cult always make sure that they remain hidden from sight, using a Hive’s old subterranean infrastructure to hide; they only come out for two things, either to expend their brood or for food, and in both cases people start to disappear.
Eventually the brood will grow and in the end you will have a large family, fiercely loyal to each other, they will also try to corrupt the mortal population and convert them to worshipping the Patriarch as a god. The cult will infiltrate all layers of society and eventually topple the ruling status quo and planetary defences and sound in the planets doom as the hive fleet enters orbit.
This document is completely unofficial and in no way endorsed by Games Workshop limited or Fantasy Flight Games.