Literally, the development of differences, it is the process by which cells become increasingly different and specialized, giving rise to more complex structures that serve particular functions in the adult organism (e.g., embryonic cells develop into bone, muscle and nerve cells).
See Cytokines, Dependent differentiation, Determination, Development, Developmental biology, Differentiation (general), Differentiation (specific), Ectoderm, Entoderm (or endoderm), Epigenesis, Equipotentiality, Fibroblast growth factor (FGF), Growth, Growth factors, Mesoderm, Neural tube, Organizer (embryology), Methylation, Permissive interaction, Preformationism, Progenitor cells, Proliferative cells, Self-differentiation, Stem cells