YA or Young Adult literature is fiction aimed at an adolescent audience. YA SF is a somewhat vague category aimed at kids too young for ordinary science fiction, but too old for reading children's books, roughly ages 12–18. In the 1950s and ’60s, the term juveniles was used for this category.
YA is mainly a marketing category, since many adults read YA books with pleasure, and many teens read books meant for adults.
In theory, YA SF is not as complex in style as adult SF, but can deal with equally important issues. In some cases, the label seems to be more associated with the age of the protagonist than the reading level. However you define it, YA SF is a cornerstone of the field, and some of our best and most famous writers have done their best work in YA. Robert A. Heinlein is a leading example; others include Andre Norton, Madeleine L'Engle, Ursula K. Le Guin and John Christopher.