I saw The Martian (the movie) about a year ago. At the time, I wondered: why bring a botanist to Mars? and speculated that it might be answered in the book. And it was, right on page 12! Good for Andy Weir, the author, for clearing that up.
Every sentient being knows the basic plot, but anyway: through a disastrous combination of events, a lone member of the Ares 3 manned mission to Mars, Mark Watney (the botanist), gets left behind while the other crew members make an emergency return to Earth. Mark is more alone than any human being has ever been, and has only the remnants of the mission to help him survive.
There are numerous ways Mars can kill you, and Mark deals with most of them. They range from passive (running out of food, water, or air) to spectacular (explosive decompression, surprisingly treacherous terrain, …). Mark deals with these issues with a combination of humor, bravery, and amazing geeky resourcefulness.
One thing, though: you're exposed to a lot more radiation on Mars than we are on Earth. As near as I can tell, neither the book nor the movie mentioned this at all. But I could have missed it. It definitely falls into the "not much Mark can do about that" category.
Reading the book got me thinking about the general differences between books and movies based on those books. It's very rare (in my experience) that both are high quality, and The Martian is one of those rarities. (Another: Lord of the Rings.) I kept noticing differences: book-Mark (heh) deals with a few more disasters than does movie-Mark. And the book is littered with far more f-bombs than is the movie, understandable for the movie's PG-13 rating. (Annie Montrose, the NASA spokesperson played by Kristen Wiig in the movie, is particularly potty-mouthed in the book.)
Bottom line: the book is very good, and it doesn't matter much, enjoyment-wise, if you see the movie first.