Twilight was written by Stephanie Meyer.
The main difference is the three vampires being around in Forks. In the book, they only show up in the area when the Cullen's and Bella play baseball. In the movie, they are there since around when Bella arrived, and are constantly killing locals, causing Charlie to investigate and a whole new subplot to occur.
In the book, vampires glow in the daylight so much that they can't even go out in public without the most oblivious person seeing this phenomenon. In the movie, they kind of have some glittery stuff on their skin, no more than if a regular person put some glitter on themselves.
Bella and Charlie never like to go out to eat. Bella always cooks. But in the movie, every meal was in a restaurant.
In the movie, Jasper was just portrayed as "that weird guy, don't worry about him, we don't think he's so important."
In the book, Jasper has the ability to manipulate people's emotions. But in the movie, Edward never mentions what Jasper's abilities are. When Bella asks if other people in his family can read minds, he just mentions Alice's abilities.