Simply, in my observation, spiritual formation is not too different from discipleship in nature. I think the difference would simply be that “spiritual formation” evokes fertile images of growth, slow maturation, and gentle becoming. It is a focused attention upon the heart – what is being stirred in your affections; what your minds attention is focused upon. In my view, “spiritual formation” is about your betterment, your sanctification in the most radical ways of which takes place in the most ordinary places.
Like unto it, discipleship is done with one another in community. You have a discipler. You have someone who is coming along side with you and helping you grow. Spiritual formation is an inward discipline; discipleship is an outward discipline. That being said, I think there is a ton of room for disagreement here… the point is that we are called to follow Jesus.
Thus, this is how I would define spiritual formation: the maturing process of a follower of Jesus, so that they might be sanctified.