Developmental psychology is the branch of psychology that studies the physical, cognitive, and social changes of humans throughout their life cycle. Some argue that developmental psychologists study changes over time which all psychologists study, not just developmentalists. However, the difference is that the topics studied by developmental psychologists revolve around the maturation and ageing process; what affects it and what it affects.
For example, a developmental psychologist and myself may each conduct a study addressing how children of different ages perform on a particular test. The developmental Psychologist would be concerned with the differences between the age groups, why they performed differently, what developmental issues may be the causal factors in the differences, etc., while I may explain the differences in terms of the test, not the developmental differences of the children (i.e., the test is age-appropriate, can we use it on people of other ages, what does having an age difference mean on whatever the test actually measured, etc.).