An unverified Wikipedia article says that "the Pilates method seeks to increase the strength, flexibility, and control of the body." The "core" is emphasized a lot. Basically your abs are continuously flexed throughout the workout. I really enjoyed my first 50 minute session. It was relaxing, different, and I could feel my body being worked. (And I looked around during certain exercises... I wasn't alone in my inflexibility!)
I found this image after searching for "pilates and outdoors." This in Colorado.