So, with the hope of not spoiling everything for everyone, let me just say:
In the movie, the director (of the ballet) says that Swan Lake has been done to death. But not the way he's going to direct it. Stripped down to its essence. Etc. Interestingly enough, when you see bits and pieces of the ballet, it looks pretty typical--from what I can tell, it's still a ballet, with ballet costumes and ballet dancing. I won't say definitively that they didn't do anything new with it, but from the snatches of the dance that you see, it doesn't appear that he's done a whole lot to change the feel of the ballet.
When you look at the movie as a whole, that's exactly what's happened.
Black Swan takes the concept of Swan Lake (innocent girl falls prey to both magician and prince, and is destroyed by her rivalry with the black swan) and presents it in a modern setting. The movie basically tells the story of Swan Lake twice, once in the ballet form, and once through the destruction of Nina through the forces and personalities swirling around you.
I'll leave you to draw your own conclusions of which characters in the movie reflect which characters in the ballet (some are more obvious than others), and to find the parallels if you want to see it. This movie is not for the faint-hearted; in fact it took me a couple days after I'd seen it to decide for certain if I liked it. In the end, though, I do think it's fascinating and raises enough questions to be worth watching.