Based on the Collins English Dictionary, a subgenre is "a category that is a subdivision of a larger genre." (Source) Adventure/thrillers is kind of a catchall category that covers books that are action-packed, very engaging, and usually fast-paced. There is however a subtle difference between Adventure and Thriller.
Adventure books tend to deal with some sort of journey. Sometimes the character is lost or finds themselves in a survivalist situation while other times they're just out exploring the world. Typically the settings of these books are exotic and not necessarily in just a tropical sense. A lot of the time we get to see a whole other side of nature in these books through the eyes of someone having to experience a natural event or combat a natural disaster.
Thriller books tend to have a component of mystery. It doesn't fall under mystery per se because instead of a problem-solving approach to the action, thrillers use suspense and tension. These books play on the reader's emotions and senses while allowing them to role play the scenario at hand. A lot of spy novels would fall under this category.
Here's what Anthony Horowitz had to say about his inspiration behind the Alex Rider series:
He found solace in boyhood in the escapism of the James Bond films, he says. He claims that his two sons now watch the James Bond films with the same tremendous enjoyment he did at their age. Bond's glamour translates perfectly to the 14-year-old psyche, the author says. "Bond had his cocktails, the car and the clothes. Kids are just as picky. It's got to be the right Nike trainers (sneakers), the right skateboard. And I genuinely think that 14-year-olds are the coolest people on the planet. It's this wonderful, golden age, just on the cusp of manhood when everything seems possible."
I think one of the better thriller books I've read recently is Stolen by Lucy Christopher. This book, written in the form of a letter to her captor, follows the events that happen to Gemma after she is kidnapped from an airport. We never really get a complete explanation as to why she was taken & we definitely don't get a pretty ending. Throughout the entire novel you're trying to piece things together & think about what's still to come. It's a fantastic read that doesn't take very long, so if you get a chance you should really think about picking it up.
I hope that helps to break down these two genres. I find myself more attracted to adventure books over thrillers simply because I seem to like survival books. Which one do you prefer? Check back next week for my review of Hatchet!