At its core, the film version and the book version of Silver Linings Playbook are essentially the same. The story is about a protagonist named Pat who has just come out of a mental health facility. His sole purpose is to be reunited with his estranged wife, Nikki. The main characters from the book and the film are all essentially there, including Pat, Pat's parents, and the widow (and future love-interest) Tiffany. Also, the common theme that there is a "silver lining" to every difficult obstacle in life, runs through both the book and the film.
The biggest differences from book to film of course translate immediately in characterization. It is nearly impossible to portray the first person narrative (through the eyes of Pat) of the book to an objective camera lens in a film. Many critics of the film (and lovers of the book) were disappointed in the lack of complexity of Pat's film character compared to who he is in the book. This is understandable. The film is unable to fully reveal Pat's personal glimpses into his own mental illness that the book makes so natural through the first person narration. Other critics suggest that Pat's parents are also much more complex in the book than in the film.
One major plot difference between the book and the film is the ending. In the book, Pat finds his ex-wife Nikki and views her from afar, realizes she has a new life without him, and walks away from the scene with a feeling of peace about the situation. The final scene of the book is of Pat and Tiffany confessing that they need one another, and the audience is left with the feeling that he and Tiffany end up together. The film, on the other hand, ends with a confrontation between the three at a football game. The specific interaction between Pat and Nikki is left ambiguous, but the audience sees Tiffany storm off in jealousy and watches as Pat runs to catch up with her, pledge his love for her, and the scene ends in a romantic kiss.
Book and film lovers agree that both depictions of the story end on a positive note, but most critics confess that book tied up the loose ends in a better way.