Young Adult is exactly what is sounds like - books for young adults. However, not only do teens love young adult books. They're very popular with tweens and adults too.
The common theme of young adult books revolve around a protagonist's growth; they solve a problem, grow as a person, or both. The main character of a young adult books is always between the ages of 12-19. Other than that, young adult books have at least one of the following themes: friendship, first love, relationships, or identity.
Although the young adult genre was established during the 1800s, it has recently seen a resurgance in popularity thanks to author's like J.K. Rowling's and Suzanne Collins.
Other notable or genre defying young adult books include:
Speak by Laurie Halse Anderson
The Fault in Our Stars by John Green
Twilight by Stephenie Meyer
Percy Jackson and the Olympians series by Rick Riordan
Smile by Raina Telgemeier
The Hate U Give by Angie Thomas