Although no exact distinction exists between children's and adult literature, you can generally categorize as children's literature any text written for a target reader below the age of 14 or selected by kids to read for themselves. The Library of Congress defines children's literature as books that serve as "information or entertainment of children and young adults. It includes all non-fiction, literary, artistic genres and physical formats."
With some overlap, the publishing industry loosely divides children's literature into categories by age, with picture books and picture storybooks for readers up to age 8; easy readers for ages 5 through 9; chapter books for kids 7 to 10; middle grade for ages 8 to 12; and young adult books aimed at kids over 12. Of course, children develop reading skills at different rates, and may read above or below their age-range. Age-based distinctions generally play no part in literary choices for adults, who base their preferences on genres, plots and authors.
Picture books convey their narrative message through illustration rather than words, introducing young children to concepts such as numbers, letters, shapes or things. Picture storybooks, on the other hand, usually incorporate traditional narrative prose alongside strong imagery, combining the two media to convey a story, and can be written for people of all ages.
Within the genres of historical fiction, realistic fiction and modern fantasy, books written for young readers are termed "chapter books," due to the way they segment and build their narrative denouement. Each chapter is sufficiently concise to hold the reader's attention but still advance the plot. Chapter books often feature an age-appropriate central protagonist who plays a pivotal role and stands as a role model for the reader. The concept of building a segmented narrative also appears in adult literature, manifesting as book series that continue the plot across multiple volumes.
Traditional literature -- stories passed down through the generations -- range from fables and fairy stories to myths and legends. These tales introduce children to the wider concept of narrative and its many genres, spanning folklore, sci-fi, fantasy, historical fiction and realistic fiction, as well as poetry. Traditional children's literature seeks to reflect and confront issues its readers deal with in their own lives. It performs the same function for adult readers, as recognizable situations and sympathetic characters draw readers into a work, irrespective of age.
Young Adult, or "YA," fiction ranks as a true crossover genre that appeals as much to the adult demographic as to the teen market. Thematically, YA novels span genres and take multiple forms: straight prose or verse, or graphical depiction in cartoon or comic form. Popular motifs include paranormal creatures, such as werewolves, vampires, angels or demons, and dystopian futures in which the hero protagonist comes into conflict with "the system." Technology often plays a central role in YA novels, frequently as an omnipresent force of control and subjugation.