Jacob is one of Bella's oldest friends that she knew from visiting her father, Charlie Swan in the summer. When Bella flirts with him to get information on Edward Cullen and his family, he introduces her to the idea that Edward is a vampire.
After months of depression over Edward's leave, Bella discovers she can hear his voice when in dangerous situations. Originally, she seeks out Jacob to help fix two motorcycles, but soon she finds friendship with him and slowly begins to come out of her depression. Jacob is one of an ancient line of werewolves, a member of the Quileute tribe that has forever been the mortal enemy of vampires. At first Jacob doesn't believe the legends, and is frightened and disturbed by the influence a young man on the reservation, Sam Uley, has on some of the Quileute youth. But when Jacob himself makes the change in response to a new vampire threat, he learns that the Quileute legends about werewolves and vampires are indeed true. He becomes busy patrolling the forest with his pack, searching for the vampires in the area who are killing local hikers. When Bella is caught by the vampire Laurent alone in a clearing, she is saved by Jacob and the pack, who manage to destroy Laurent. When Bella impulsively jumps off a cliff and almost drowns, Jacob pulls her out and saves her. After Edward mistakenly believes Bella to have died, she and his sister, Alice, rush to Italy stop Edward from killing himself, leaving Jacob hurt and angry. Jacob is disgusted by Edward's return and by Bella's willingness take him back after he left her; he is also outraged and upset when he learns of Bella's plan to become a vampire. Jacob reminds Edward of their treaty, which states that the Cullens are not allowed to bite a human. This means that if one of the Cullens turns Bella into a vampire, the treaty will be broken and the werewolves will start a war.
Jacob attempts to distance himself from Bella by not returning her phone calls and refusing to see her, enraged by the fact that Bella plans to become a vampire after graduation. At the same time, Edward forces Bella to keep away from the tribe, doing as much as disabling her car engine to prevent a visit. Later, Jacob visits Bella and Edward's school to discuss the vampire Victoria's return, and he tells Bella that he misses her and that he believes they can still be friends, much to Edward's annoyance. With Edward's approval, Bella begins to visit Jacob on a regular basis. On one of these visits, Jacob tells Bella that he loves her and that he wants her to choose him over Edward. He forcefully kisses her, much to her displeasure, and she reacts by punching him in the jaw. Jacob is unharmed but the impact of the punch breaks Bella's hand, and she is furious with him. Jacob brings Bella home and is congratulated by Bella's father, but threatened by Edward. Jacob's pack of werewolves and the Cullen family join forces to defeat Victoria and the army of vampires she has created. Before the battle, Jacob overhears Edward and Bella discussing their engagement, angering him greatly. After some talk, Jacob threatens suicide and Bella, in an attempt to stop him, tells him to kiss her. Afterwards, Bella comes to the realization that she loves him as well.
Information from: http://en.wikipedia.org/wiki/Jacob_Black