While pedagogy refers to the teaching of children, where the teacher is the focal point, andragogy shifts the focus from the teacher to the learner. Adults learn best when they have control over their learning.
The first known use of the term andragogy was by the German educator Alexander Kapp in 1833 in his book,Platon’s Erziehungslehre (Plato’s Educational Ideas). The term he used was andragogik. You can read more about this first use at Andragogy.net.
Check out Gwen's definition of andragogy.