Developmental psychology is the study of how humans grow and, well, develop across the lifespan. Across infancy all the way up to old age, human beings experience a great deal of change, growth, and then decline. These changes encompass physical (i.e., learning to crawl in infancy), cognitive (i.e., learning how to apply logic to solve real-life problems), and social development (i.e., learning how to share and cooperate with peers). Up until fairly recently, developmental psychology used to be focused primarily on the study of infancy and childhood. Maybe it was the aging of the Baby Boomer generation that prompted the field to grow up (pardon the pun), but now developmental psychologists produce research and scholarship on life stages all the way up to old age.
As a developmental psychologist, I was interested in adolescence, young adulthood, and transitions between stages. Examples of research questions that developmental psychologists strive to answer: What academic skills developed during high school predict high GPA in college? How does exposure to violence on TV as a child affect perceptions of dating violence in adulthood? Does excessive video game playing affect adolescent socialization skills? How does socialization with peers affect cognitive decline during old age?
If your research interests focus on a particular life stage or transitions between developmental periods, then developmental psychology would be a good focus for graduate study. To be certain if this is an area of interest for you, take a look at emerging research in the field. Your university library should have a collection of academic journals, such as Developmental Psychology, Child Development, or Journal of Adolescence. Flip through a couple of issues and note topics which catch your attention. These are potential areas of research interest. Also take note of article authorship and their university affiliation. Just like that, you can identify a potential faculty mentor and quite possibly, the right graduate program for you!