In short, Eclipse is the third book in the Twilight series. In this book, Bella is approaching graduation, after which she plans on being turned into a vampire. Her friend Jacob does not want this to happen, and he reveals that he is in love with Bella, providing her with a choice between Jacob or Edward. During this strife, the vampire Victoria has created a vampire army in order to kill Bella. The Cullens and the werewolves, who are natural enemies, call a truce and band together in order to stop the newborn vampire army.
Enotes has a full summary of Eclipse posted under the "study guide" tab above. Enotes has provided a comprehensive summary, analysis of characters and themes, and a background of this book's literary significance.