Rasmus is right lich, its seriously ALOT of info to even begin to try and get across. If your interested in this then you really need to research it yourself and then come with the questions, you cant just ask for a fluff generalization because we would be here all year trying to flesh out such a generalization.
Basically Genestealers are Tyranid creatures that operate a bit like a disease, they infect other organisms (especially humans) with other Genestealers, and they generally cause alot of damage.
A Genestealer cult is a body of Genestealers, Hybrids and Humans that are held together like one giant mutated family, they operate in secrecy and try and cause mayehm and discord before a Tyranid Hive fleet arrives to eat everything on the planet.