Bouldering is a form of mild rock climbing, where you basically free climb, but not to great heights. Typically you would not have a rope or harness and you do more a side to side movement across the rock and keep closer to the ground. It is truly a great workout as it will work parts of your body that you might not use every day, so you can build some great endurance. On a recent trip to CO - I went bouldering in the town of Boulder! Which I found to be fun. Boulder being so close to Denver made this a great day outing.