"The Indians in this frozen land tell of a ghost dog which runs at the head of a wolf pack. - They are afraid. For it has more strength than any wolf, more cunning than any dog. No one knows from whence he came or why he stays."
Gold was found in Alaska, the rush to obtain it required a strong constitution and many dogs to do the work that horses usually did in the states. The environment bred harsh attitudes. Also, in the testing of one's mettle, one finds his/her true potential.
Buck (larger than most dogs) goes through many lives, trials, and tribulations finally realizing his potential. On the way he learns many concepts from surprise to deceit, and cunning; he also learns loyalty, devotion, and love. As he is growing he feels the call of the wild.
You do not have to have read the book to follow the story. However, being a film, much is accelerated and leaves less time for character and drama buildup. A few characters are removed but the main characters do not have to be modified to fill in the gaps.
For those that have read the book, you will notice the narration is word-for-word extracted from the book. You may also recognize the narrator's voice as that of Richard Dreyfuss. The dialog was modified to cover much of the narration that was left out or the descriptions that were not portrayed.
A few stories were left out or modified for brevity. The main difference is a lot of the blood and guts were replaced with inferences and a story where he gets the stuffing beat out of him is replaced with a threat of being shot. My only disappointment was leaving out the part where Buck, as wild as he becomes, periodically visits the place of John Thornton's demise.