comparison

Comparison with the source material
Jackson, Walsh and Boyens made numerous changes to the story, for purposes of pacing and character development. Jackson said his main desire was to make a film focused primarily on Frodo and the Ring, the "backbone" of the story.[11] The prologue condenses Tolkien's backstory, in which The Last Alliance's seven year siege of the Barad-dûr is a single battle, where Elendil is simply killed by Sauron, and his defeat is a quick swipe from Isildur using the broken Narsil. Sauron is shown to explode, though Tolkien only said his spirit flees.[12] Isildur keeps the One Ring as a commemorative, but is not corrupted by it as described in the narration. He is advised to destroy the Ring, but nothing is said of him and Elrond actually going to the Cracks of Doom as shown later on with Elrond and Gandalf's discussion.[13]
Events at the beginning of the film are condensed or omitted altogether. In the book, the time between Gandalf leaving the Ring to Frodo and returning to reveal its inscription is 17 years, which is compressed for timing reasons.[14] Frodo also spends a few months preparing for his journey to Bree which is compressed to a day, to increase dramatic tension. Also compressed is up to when Frodo and Sam leave Bag End and the meeting of Merry and Pippin. Characters such as Tom Bombadil are left out for plotting reasons as well as increasing the threat of the Ringwraiths. Such sequences are left out to make time for Saruman, who in the book only appears in flashback until The Two Towers. Gandalf's capture is also expanded with a fight sequence. Saruman's role is enhanced: he is to blame for the blizzard on Caradhras, a role taken from a spirit in the book.
A significant new addition is that Aragorn must overcome his self-doubt to claim the kingship. This element is not present in the book, where Aragorn intends to claim the throne at an appropriate time. He reforges Narsil immediately when he joins the Fellowship, but this event is held over until The Lord of the Rings: The Return of the King film. All this was done because of Peter Jackson's belief in "character growth", the idea that every character must change or learn something over the course of the story. Arwen Evenstar also has a greater role in the film, replacing the book's character of Glorfindel in rescuing Frodo, a surviving element of her earlier warrior princess incarnation in the script that survived due to being shot early on. Elrond is also different from his counterpart in the printed novel; in the film he doubts the strength of Men to survive without a King. Jackson also skims the Council of Elrond into establishing the Ring quest, by moving exposition from the chapter into earlier parts of the film.
The tone of the Moria sequence was altered. Although the Fellowship only realize all the Dwarves are dead once they reach Balin's tomb, the filmmakers chose to use foreshadowing devices instead. Gandalf says to Gimli he would prefer not to enter Moria, and Saruman has a telepathic communication with Gandalf, and also reveals an illustration of the Balrog in one of his books. The corpses of the dwarves are instantly shown as the Fellowship enter Moria.[15]
The book simply stops in terms of dramatic structure, as Tolkien wrote it as a single story published as three. Jackson's finale is played as a climactic battle, to which he introduces the (unnamed) antagonist referred to as Lurtz in the script. In the book the battle leading to Boromir's death is told in flashback in the second volume, but in the film their encounter is shown in real time. Adding to the ending before the wait for the next film, Aragorn is aware of Frodo's decision to leave.
source:Wikipedia