I just went over this distinction in a Mindfulness-Based Stress Reduction class. We learned that most of us get used to just reacting, which tends to be unconscious or automatic and can produce stress. Whereas responding is more measured and thoughtful, using the power of an observer position that can see the larger picture and can take a breath before "reacting." We learned that not only do we do this with things and others, but we also carry it on inside ourselves, reacting to thoughts and feelings, instead of responding. That made a lot of people in the class really change the way they were and improve their lives tremendously. It's an important distinction. Hard to describe though. But you know it when you see it or do it.