I dont understand the storyline of the movie. HELP!

Ok, i watched the movie n i thought it sucked... I told my friends n everyone keeps saying that "It sucks when u dnt understand it but if u understand it, it is a really good movie"

So, can someone please tell me the story? I really dnt understand. Is it some kind of fantasy movie or what?

Hw the hell did Nina kill herself? Im really confused!