There are many differences between breaking dawn the book and breaking dawn (parts 1 and 2) the movie. Some are blatantly obvious while others you have to know where to look to even be able to spot them.
I'll start with the book and part one of the movie. Edward and Bella had talked about her staying human for a full year before she turned so she could attend Dartmouth, But in the movie there was no talk at all about collage. In the movie Edward shares with Bella his rebellious vampire period the night before the wedding while in the movie it is only told is the book Twilight. The day of the wedding comes and it goes on quite beautifully. Many speeches were given in the movie yet none were described at all in the book. After the wedding Edward and Bella leave to go to the place where they were said to have their honeymoon. They were their 17 days after the wedding when belle discovers she was pregnant. In the movie it had only been 14 days since the wedding. She had decided to keep the baby and becomes very ill and forced to have the baby removed from inside of her because if she wasn't Bella would have died. Edward roomed the baby and had no other choice but to turn her. Bella awakes from her transformation in the book wearing a silk blue dress and stiletto heals. In the movie she is wearing a blue dress but is also barefoot. She is then presented to her daughter which they names Renesmee. Edward showed Bella Renesmee's powers and got to hold her for a bit. It was suggested she should hunt so Edward and Bella left. In the movie Bella encounters a mountain lion and a deer at the same time. After she had smelled a human and went after him. Edward stopped her. In the book the animals were hunted separately and she never goes after the human she smells. They get back and soon after Jacob arrives and Bella gets into a fight with him because of the fact that he had imprinted on her daughter and had given her the