  Galaxy morphology is a product of how galaxies formed, how they interacted
with their environment, how they were influenced by internal perturbations,
AGN, and dark matter, and of their varied star formation histories. This
article reviews the phenomenology of galaxy morphology and classification with
a view to delineating as many types as possible and how they relate to physical
interpretations. The old classification systems are refined, and new types
introduced, as the explosion in available morphological data has modified our
views on the structure and evolution of galaxies.
