Young adult is generally fiction that addresses itself to teenagers. Mostly for teens from 12 to 18.
It defines itself as character exploration through themes of youth such as love, high school, sex, family, friendships, and the definition of growing as well as what it implies.
They can take place in the real world or in fantasy realms and are usually turned into blockbusters that speak to the X generation. YA is a genre that allows the transition between child and adult to be more seams.