Developmental biology is concerned with the study of how a multicellular organism develops from its early immature forms (zygote, larva, embryo, etc.) into an adult. This field heavily interrelates with other biomedical sciences such as genetics and cellular and molecular biology and is concerned with everything from evolutionary theory to the practical applications of how to prevent human birth defects.
Fruit flies, sea urchins, mice, and amphibians such as axolotls are commonly used in developmental biology research.
This writeup is based on work I did for the science dictionary at http://biotech.icmb.utexas.edu/