Jella

"Bella actually falls in love with Jacob in New Moon."

- Stephenie Meyer, the Twilight Saga's author

Jella is the het ship between Jacob Black and Bella Swan in the Twilight fandom.

Twilight
Jacob and Bella meet in Twilight. They go for a walk down the beach, and Bella flirts with Jacob to persuade him to tell her some of his tribe's legends. There isn't any romantic feelings between them in the first book, however.

New Moon
Jacob and Bella spend much more time together in New Moon. Bella was dating Edward Cullen, and when he left her, she was heartbroken and became depressed. She mentions that Jacob was the only person that made her happy; therefore, they spent a lot of time together. At one point, Bella invites some friends from school, along with Jacob, to a movie with her. However, only Jacob and a boy named Mike end up coming. Both boys try to hold Bella's hand during the movie, but she doesn't hold either. When Jacob becomes a werewolf, he ignores and spends all of his time away from her. Instead of being hurt, Bella is worried for him and tries calling many times. When she learns his secret, they still remain friends.

Eclipse
In Eclipse, Jacob and Bella spend some time together throughout the book. One day, Jacob reveals to Bella that he is in love with her and that he wants her to choose him over Edward. She is surprised by this declaration. Then, Jacob kisses Bella, and she reacts angrily, punching him in the face (which, in turn, breaks her hand). Edward makes him promise that he will never kiss her again (unless she asks him to). Later, Jacob finds out that Edward has proposed to Bella and becomes hurt and angry. He threatens to go fight someone and get himself hurt, to which Bella responds with the only thing that would make him stay: she asks him to kiss her.

Breaking Dawn
Jacob receives an invitation to Bella's wedding and becomes angry. He comes to the wedding at the end, though, and Bella dances with him. She reveals that on their honeymoon, she and Edward would have sex, which angers Jacob because Edward is a vampire and is therefore incredibly strong and could easily injure her. Some of Jacob's friends have to hold him back from attacking Edward. Later, when Jacob finds out that Bella is pregnant, he is happy for her. Bella momentarily dies during her baby's birth, and Jacob becomes distraught, fleeing from the house where she was giving birth and crying. Later, he goes back inside with the intent of killing the baby that killed Bella, but as soon as he sees her, he imprints on her (which is a special thing that werewolves do, meaning that they meet their soulmate for life). As he is destined to fall in love with Bella's daughter, he just stays friends with Bella and nothing more.

Fanon
Ships between Jacob & Bella and Edward & Bella are the two most popular ships in the Twilight saga fandom, and people generally take the side of 'Team Edward' or 'Team Jacob' (though some people in the fandom are on 'Team Neutral').

Wikis
Bella Swan and Jacob Black on Twilight Saga Wiki