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I've heard of them, but what are they?

Wiki page: http://wh40k.lexicanum.com/wiki/Genestealer#.TrQMDUOXuso

Gene stealers can infect people, which causes them to want to mate. When they mate they will have first generation gene stealers, gene stealer hybrids, and they (and their mate) will be hypnotically compelled to care for them, fanatically protecting them and ignoring their mutations.

This continues for, I think, 4 generations, each generation becoming more and more mutated, and each generation being totally, fanatically devoted to the brood.

The first generation has lesser, mainly vestigial mutations and can pass for human, and often focus on infiltrating the local society. The second generation must typically remain covered to pass for human, but are stronger and more dangerous. The third is a full on abhuman, extra limbs, claws, and typically stay at the nest, guarding it. The fourth generation are full on gene stealers, who can then infect new humans. A "Brood Lord" will also be spawned, a monsterously strong psychic creature who leads the cult.

This 'cult' attempts to infiltrate society and government, sowing discord and chaos. They also attempt to infect people going off world, so as to further spread the taint to other worlds. Each member is part of the "brood mind", a mini hive mind to which they are totally loyal.

The brood mind also acts as a psychic beacon, drawing hive fleets towards the world when the cult grows large enough. When a hive fleet arrives they will attempt to sabotage the planets defensive effort, an easy thing to do since a large portion of the military is likely to be cultists at this point.

If all goes according to plan the hive fleet will be able to devour the planet with minimal opposition.

A cult led by a genestealer, trying to grow and wait for the arrival of a Hive Fleet for salvation. Still part of the current fluff.
In Rogue Trader days, genestealers and genestealer cult miniatures were abundant and more numerous than genuine Tyranid miniatures (starting with termagants, Zoats and Warriors). With the first Tyranid Codex, Genestealers and Genestealer Cult were included as part of the whole Tyranid race and as playable units. Later they got a half official list. Several updated fan-made lists floating around.

There the essence of cool in the old days. Their army list had "Armored Limousines" in it, lol.

Otherwise riplikash and the Lexicanum sum it up pretty nicely. I used to play Genestealer Cult in small points tournaments for 2nd Edition.