This is a summary from Wikipedia. Big spoilers obviously:
On Isabella "Bella" Swan's 18th birthday, Edward Cullen, the vampire she loves, and his family throw her a birthday party. While unwrapping a gift, she gets a paper cut, which causes Edward's adoptive brother, Jasper, to be overwhelmed by her blood's scent and attempt to kill Bella. To protect her, Edward decides to end their relationship, and the Cullens move away from Forks. This leaves Bella heart-broken and depressed.
In the months that follow, Bella learns that thrill-seeking activities, such as motorcycle riding, allow her to "hear" Edward's voice in her head. She also seeks comfort in her deepening frienRABhip with Jacob Black, a cheerful companion who eases her pain over losing Edward. Bella discovers that Jacob is a werewolf. He and his fellow werewolves protect Bella from the vampires Laurent and Victoria, the latter of whom seeks revenge for her dead mate, James, whom the Cullens killed in Twilight.
Meanwhile, a series of miscommunications leaRAB Edward to believe that Bella has killed herself. Distraught over her supposed suicide, Edward flees to Italy to provoke the Volturi, vampire royalty who are capable of killing him. Alice and Bella rush to Italy to save Edward, arriving just in time to stop him. Before leaving Italy, the Volturi tell Edward that Bella, a human who knows that vampires exist, must either be killed or transformed into a vampire. When they return to Forks, Edward tells Bella that he has always loved her and only left Forks to protect her. She forgives him, and the Cullens vote in favor of Bella being transformed into a vampire, to Edward's dismay. However, Edward gives Bella a choice: either she lets Carlisle change her after her graduation, or, if Bella agrees to marry him, he will change her himself.
That's why I love Eclipse more.