Celie's sexuality. Is Celie a 100-percent straight woman? In the novel, she's not. She has an affair with the singer Shug. What about Shug? Is Shug a 100-percent lesbian? It'd be hard to argue that she is, given that she married a man and falls in love with another young man.

Anyway, this is all to say that in Walker's novel, Celie has a fluid sexuality, while in the film it's much more stagnant and static.

It might be interesting to talk about the film's choice to all but clip Celie and Shug's romantic relationship within the context of bisexual erasure. It might also be worth wondering: If Celie and Shug were white women, would Spielberg have been more comfortable clearly showing their affair in his film?

In addition to examples given in previous answers, I would say that a major difference between the book and the movie directed by Steven Spielberg is that the movie essentially cuts out the entire section of the book where Celie and Shug live in Tennessee together. We see Celie finally leave Mr.—, we see her attend her step-father's funeral and inherit his house, and we see her later on after she's started her successful business, but we do not see what happens in between.

In Alice Walker's book, we get to see Celie as she lives in Tennessee and develops her company, Folkspants, Unlimited. This business gives her financial independence as well as a way to express herself.

The movie also completely cuts out the emotional plot point (in Letter 83) where Shug breaks Celie's heart by telling Celie that she also wants to have a relationship with a young man called Germaine. Because of this, Celie decides that she can no longer live with Shug in Nashville, and returns to Georgia. Getting over this pain helps Celie to move on from her dependency on Shug, as well as find common ground with Mr.—.

One other significant difference between the book and the movie is that, in the movie, Shug Avery is made somewhat more vulnerable than she is in the book.  In the book, she comes to the home Celie shares with Mr. _____, sick and in need of nursing; it's the same in the film.  However, the movie creates a conflict between Shug and her father, a minister who disapproves of her lifestyle and career choice as a performer and singer.  In the movie, Shug seeks his approval, something which he continues to deny her.  And she moves on, but it seems to be a soft spot for her forever.  In the book, there is no such conflict or vulnerable soft spot.  

Further, the pants that Celie learns to make are a major symbol in the book, and she makes pants with all kinds of fabrics for everyone: both men and women.  The symbolism seems to be somewhat lost in the film.  Much less focus is placed on Celie's business in the movie than it is in the book.  

• Alice Walker's novel The Color Purple is much darker and more complex than Speilberg's adaptation, which develops the comedy and the musical aspects more.  In the book, the men are meaner, crueler, more abusive, more sexist.  In the movie, they seem more like childish clowns, especially Mr. ____.  The book reads like tragedy and ends up like comedy only at the end.  The film introduces the comic/musical aspects much earlier and develops them throughout.
• Celie is a writer in the novel: she writes to God.  It's an epistolary novel.  Letters to God is the impetus of how the novel beings, as confession.  There's no such writing or confession in the movie: it's interior monologue done as voice over.  Speilberg only shows her writing to Nettie, not to God.

Some other, smaller differences:

• There's no quilting in the movie.  It's a major motif in the book.
• The movie shows more female's kissing (homosexuality) than the book (there's only Shug and Celie), which takes away from the power of the sexual experimentation.
• Again, the music is a major character in the movie (Quincy Jones was brought in to do the score), and there's more hymn singing and juke-joint cross-cutting to make it seem more like a musical than a novel.  Obviously, this is a major reason why it becomes a musical on Broadway later.

How many times have you read a book and gone to see its film rendition and found major differences? The film was not how you imagined the story to be while reading the book. Well, the same can be said of the movie and novel The Color Purple, written by Alice Walker. While the movie is very moving and a spectacular work, directed by Steven Spielberg, there are nuances that any reader would find different to how they imagined reading the story for themselves.

Three differences between the book and movie are the following: in the book, Celie addresses her diary entries/story to God, while the movie covers a more general theme of salvation and doesn't include this aspect of her narration; the scenes of Celie's lover, Shug Avery, seeking out her father's approval in his church are missing from the book; and for this reason, the feminist ideology that is reflected in Walker's novel is changed through these scenes where Shug bows down to male authority in seeking redemption from her father.

One of the difficulties in trying to portray a novel like this as a film is that the narration of the novel precludes an accurate reflection. Note the way that the novel is narrated from the point of view of Celie, and although the film tries to replicate this by having a voice over, narrating her thoughts, the book gives us a far deeper insight into the character of Celie than the film does.

One thing that a film always does that is different from what a book can do--which may be either a good or bad difference--is to render the narrative to an entirely visual experience (turn off the sound and you still have a meaningful experience of a visual narrative). The camera intrudes--or enters gracefully in--as a character originally absent from the narrative: the camera sees and interacts with the narrative giving it the dynamic of a character. Even a passive camera that is fixed in one place, does not move in any direction, doesn't zoom in tightly or out loosely, etc. gives a unique visual representation to the narrative the book cannot give. The significance of this is that the imagery in the film--and the memorability of the imagery--will be radically different from that in the book. The significance of this is that the viewers perception and understanding of and interaction with the narrative will be radically different. Thus it is that authors can, and do, say along with Alice Walker that the "movie is not the book."