The comments on this question have most of the information that seems readily available about the differences between the movie and the film, and I myself have not read the novel, so bear that in mind.
Here's the link to the site with the best analysis I noted.
To sum up, the primary differences are:
The number of characters in the transport to Vega; in the movie, there's only room for one. In the book, it seats five.
The character of Palmer Joss is a religious writer in the movie; in the book he's a preacher.
The end of the Cold War caused one of the novel's subplots to be dropped from the film.
There are (arguably) different motivations for some of the characters in the novel than as shown in the film.
The film apparently plays the "science-vs-religion" aspect up more than the novel.
The character of Ellie didn't face the sexism she encountered in the film.
The romantic aspect of the film does not exist in the novel.
Here's another link to a Quora question with some of the same points listed.