It has science-fiction; The Drs. Murray, parents of Meg, Charles Wallace and the twins) are scientists who are researching Time and Space. Dr. Murray takes a time trip and so do the kids.
There is also magic; a trio of "witches" shows up--Mrs. Whatsit, Mrs. Who and Mrs. Which, and they take Meg, her brother Charles-Wallace, and their new friend Calvin on an epic adventure.
It's also the story of a family with a deep trouble who nonetheless stay together, the story of a young girl who is just coming into adolescence with all the awkwardness and confusing feelings, and the story of a special little boy who is thought to be retarded by townspeople.
Meg Murray was one of my best friends growing up. She was imperfect, and loving, and confused, and wickedly smart, and astonishingly dense, and absolutely could not see the beauty of herself (both inside and outside). As a young girl who was also struggling with these things, I found solace and comfort in immersing myself into books where in "the real world" the same types of issues occurred, but that there were "greater" things going on, that she was so uniquely qualified to work on.