"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.)