A common trend among YA novel movie adaptations is to split the final book (typically the longest installment) into two movies. The Twilight saga movie adaptations hopped on this trend with the choice to split Breaking Dawn into two movies, further popularizing the move with the financial success of both parts.
Both Breaking Dawn Part 1 and Breaking Dawn Part 2 were directed by Bill Condon, who was thought to be a good choice due to his experience with high-budget blockbusters and penchant for creating movies with a fairytale feel. Stephenie Myer was involved in the production of the film, but of course, changes from the source material had to be made.
Edward's Past
In the first Twilight book, Edward tells Bella about his vigilante past. When he was a young vampire, he wanted to taste human blood, so he used his mind-reading abilities to track down murderers and rapists and kill them. He later feels like a monster for doing so and tells Bella when he brings her to his parents' home for the first time.
The first movie skips this conversation, so in Breaking Dawn Part 1, Edward tells Bella about his past the night before their wedding. "I used to be vampire Batman, see you at the altar," that kind of thing.
Bella's Dream
In the Breaking Dawn book, Bella has prophetic dreams about having a baby that she has to protect from the Volturi, but the movie changes the nature of her nightmare.
The night before her wedding in Breaking Dawn Part 1, Bella dreams that she gets married atop a pile of the dead bodies of her friends and family. This is perhaps representative of her fears about not being able to control herself once she becomes a vampire.
Reception Speeches
The first Breaking Dawn movie makes a nice addition to the book by adding in some sweet and funny speeches at Bella and Edward's wedding reception. Esme's is sweet, Emmett, Charlie, and Jessica get some laughs, and Renee's is embarrassing.
These speeches weren't in the book at all, but it's a nice reminder to Bella of what she's leaving behind and moving towards by choosing to marry a vampire. Edward also gives a speech, and fans always enjoy hearing his point of view.
Jacob's Pack
In the book, Jacob and his new pack's dynamic is explored in greater depth. Additionally, they all spend much more time in their wolf form.
The film cuts some of this for time and makes the less expensive choice to have the conversations between Jacob and Leah happen in their human forms. While the advances in CGI are impressive, nothing beats a human face when it comes to emoting.
Alice And Jacob
The chapters in Breaking Dawn that are told from Jacob's point of view show a softening of his opinion towards the Cullens. One unexpected friendship of sorts that's formed is between Jacob and Alice. Alice can't have visions of beings she hasn't experienced herself, so she only sees humans and vampires. She can't see Renesmee, so being around a pregnant Bella all the time gives her a headache.
For whatever reason, being around Jacob, another person whose future she can't see, cancels out that headache, so the two end up spending a little time together and bond over their fear for Bella and her pregnancy. This isn't included in the movie.
Jacob's Imprinting
Jacob's imprinting on Renesmee is told from his point of view in the book. He describes feeling as though every feeling he has for others in his life - Bella, his father, his sisters, his friends - is all cut away and replaced with his love for ... Bella's baby.
The movie portrays it differently, showing a very creepy CGI teenage version of Renesmee in the future.
Reactions To Renesmee's Name
In the book, Bella tells everyone about her idea of combining her and Edward's mother's names, Renee and Esme, into Renesmee, for their daughter. When she floats around this idea, Rosalie is the most enthusiastic about it, stating that it fits well because it's one-of-a-kind, just like the baby.
In the movie, it's nearly the same, but Edward is the one who is the most on board with the name Renesmee, perhaps to show how he's come around on the pregnancy.
The Cullens Vs The Wolves
In Breaking Dawn Part 1, Sam and his pack arrive after Renesmee's birth to try to kill the baby, which they see as a threat since it's half-vampire. All of the Cullens, besides Rosalie and Bella, fight with the wolves until Jacob imprints on Renesmee.
He runs out and tells Sam, who apparently respects that enough to decide not to kill the baby. In the book, there's no mention of a fight, despite the tension between the Cullens and the wolf pack.
Renesmee's Growth
In both the book and the movie, Renesmee's growth is highly accelerated, to the point where she's growing inches in weeks. By the end of the book, she looks like a two-year-old, though she's only a few months old.
The movie accelerates her growth even further. As a baby, she has a CGI face, and as a toddler, the filmmakers used an effect to mash Mackenzie Foy's face over the younger actresses. By the end of the movie, she's played by Mackenzie Foy, who is clearly much older than two.
Alice's Vision
As mentioned earlier, Alice's visions work in a very specific way, with limitations to what she can see in the future. She can't see anything if a werewolf or half-human/half-vampire is involved, because she's never been either herself. But, for the finale of Breaking Dawn Part 2, the writers bent the rules of cannon a bit to make for a more cinematic, climactic ending.
Fans who watched the movie in the theater will recall the shock of seeing Carlisle beheaded, and the ensuing deaths of Jasper, Leah, and Seth, among others. It's revealed to all be happening in Alice's vision, which she shows to Aro. This couldn't happen, but, since vampires and werewolves don't exist, fans can let this one go.