The movie is about a boy (dearest Leonardo DiCaprio, swoon and whatnot) who rises out of the past to extract bloody revenge upon the gang lord who killed his father in a massive gang war unlike anything I have ever seen. This all takes place in the dirty streets of mid eighteen-hundreds New York, so most of the killing is done with knives.
Daniel Day-Lewis plays a character named William Cutting, or Bill the Butcher, the man who killed Amsterdam's (Leo, our hero) father (Liam Neeson, playing Liam Neeson). Never have I experienced such a completely fleshed-out character; it's as if Lewis created an entirely new person out of nothing. He is enigmatic, violent as hell, funny as hell, and I remember the first time I saw it, my hairs would stand on end whenever he was on screen.
It's interesting. While Gangs is one of the most violent movies I've ever seen, there is no graphic violence actually demonstrated onscreen, it's all implied. True, it's implied by showing blood, but rarely, if ever, do you actually see the act. I didn't even notice this the first time around. After the first battle scene you're always on your toes because you never know when Bill's going to kill someone.
Overall, the main reason I love this movie and bought it (it's out on DVD now) is that after seeing it for the first time, it literally became a part of my waking existence for a few weeks. I assimilated Bill's mannerisms into my own, and for a while I actually walked the same stilty, spindly way he does. It was actually hard to stop thinking about the movie for a long, long time. I've watched it a couple times in the past few days, and let's say it's highly recommended.