Developmental Psychology is the scientific study of how and why human beings change over the course of their life.
A student of Developmental Psychology normally studies various developmental stages from birth till adulthood, and in some cases, till old age.
A few textbooks designed for undergraduate university students that I read, by and large cover topics such as cognition, learning, literacy (language and mathematics, reading and writing) etc. What is paramount in the advanced world that we live in today is the importance of what we learn as individuals, to live in a world that exists in such tight social, economic, legal and cultural norms. In fact, we are born in this world of norms, which we have to survive our entire lifetime. In comes our newer generation and we spend our life preparing them for the future – to survive the same system that we survived.
Nature (what we inherit) and Nurture (the environment) are an integral part of the individual in a social set up. So from my viewpoint, Developmental Psychology is quite as important as Human Development (as defined by the United Nations).