It's a novel by Alan Moore and it's amazing It's possibly the most celebrated graphic novel...ever, pretty much. If you can get hold of it then I highly recommend it to anyone.
Yes, I've heard of it. Never read it, so I just know rumors about specific content. It is one of the more well-known graphic novels. It's about a bunch of anti-heroes. People with super powers "keeping the streets clean", but with real problems of their own. I don't know a lot about it other than it is pretty gritty. Supposedly, for example, there is a scene in the graphic novel where "The Comidian", one of the "heroes" rapes a woman.
It is indeed one of the greatest comic book series of all time. Time had it on their list of greatest "Novels" of the 20th century. It is definitely steeped in moral ambiguities, although I wouldn't say that the novel is about a bunch of anti-heroes. Some of the characters would fall under that heading but many just have issues.
I never figured out what was so great about it... But for today's standards it's normal. Back in it's day I can see how it was great compared to the villain of the month comics like Superman and Batman.