For those who are totally confused......
Which there seems to be MANY of you, I will attempt to clear some things up. Granted, I did not fully understand the movie the first time I watched it either, so I will not attempt to make you look an idiot...
Cassie was struggling to live, after being in the accident. She was in a sort of "parrallel universe." Her boyfriend was alive in one, and dead in the other. That is why if you listen to his dialogue you can pick up on this. He is telling her to fight, and what not. She is basically on her death bed. She can either fight to live, or give up and die. The whole Annie/Raven/rave world is the dead world. All those people have died in reality. For example, her boyfriend says something to the effect of, "If you stay here, you'll die!" By this he is saying if you stay in the state of unconsciousness, or a comatose state, you will eventually die, and he wantes her to keep fighting to live. Another example is when Cassie "kills" the man in the pool. She talks about killing him, which in reality she REALLY did, by the car accident. Another prime example is her professor stating, "There is no answer for this kind of test," or something to that effect....that is parallelin reality, which has to do with her being in the hospital after the accident, and all kinds of tests are being run on her.
This is just some of the parts I remember, that I think will sort of clarify some things about the plot that have A LOT of you confused. Hopefully this helped some of you!