Tuck Everlasting, by Natalie Babbitt, is a novel about a girl named Winnie who meets the Tuck family in the forest. Theirs is an unusual family, compared to Winnie's, who are prim, proper and strict.
The novel has some elements of realism: Winnie is given the opportunity to closely examine her family and decide what she feels comfortable with and that which she rejects (typical dilemma for a young person); Winnie is also driven to make an important decision on her own (immortality, marriage, or no?). In addition, she must try to be objective with regard to a concept that may seem ideal to her (living forever) from the outside, but which is much different than the reality of living that "ideal" existence. Forever is a long time.
At the same time, the novel has elements of the fantasy genre as well. The premise that the Tucks live in a world outside the natural one, and they stay "forever young," and can offer the gift of immortality, takes Winnie into unexpected realms of imagination.
Though there may seem to be several choices for the label that this type of literature might be given (e.g., fantasy, etc.), I think the best genre specification for Tuck Everlasting is young adult.