Fiction books represent the stories within the Star Wars universe, also called the Expanded Universe. The adaptations of the films, the spin off novels for adults and kids all fit within this category. If it deals with the characters from the Star Wars Saga, you'll find it here.
Young Adult books are geared to younger readers between the ages of 9-12. The tend to focus on younger characters or present the material in a more simplistic way. These stories have existed since the beginnings of the franchise, but have expanded their appeal (and the continuity) through the Prequel era.
Star Wars Adventure Club