"Young Adult" -- fiction aimed at an adolescent audience. YA SF is a somewhat vague category of SF aimed at kids too young for ordinary SF, but too old to be reading children's literature. (In the 1950s, the term juveniles was used for the YA category.)
YA is more of a marketing category than an actual one, since many adults read YA books with pleasure, and many kids read adult books. In many cases, it seems to be more associated with the age of the hero or heroine than the writing.
In theory, YA SF is not as complex in style as adult SF, but can deal with equally important issues. 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, for instance.)
See also: Alex Schomburg - Andre Norton - Andre Norton Award - Bibliography - Bowman Press - Continuum 15 - Dreaming Robot Press - Earthsea Trilogy - Eleanor Cameron Award for Middle Grades - Eugie Foster - Gwyneth Jones - InConJunction 34 - Julian May - Lester del Rey - Madeleine L'Engle - Michael M. Levy - Mythcon 43 - Mythcon 44 - Neal Barrett, Jr. - Pat Cadigan - Paul French - Robert Silverberg - Sarah Pinborough - Shiny - Susan Cooper - Tamora Pierce - Ursula K. Le Guin - Ursula Vernon - Winston Juveniles - YACon - YAFA - YAFA Staffer