Morphology is the study of the basic patterns of things (Zwicky 1957). It is convenient to divide these into patterns of observed forms and patterns of behavior.
A natural first step in the study of galaxies was to classify them into a small number of "natural groups" (Morgan 1951), each of which isolates common structural features seen in photographs. Such descriptive taxonomies are useful because galaxies with similar appearances turn out to have similar physical properties. This work has led to well established schemes of galaxy classification (section 2.2), which have been successful in describing galaxy physics in two different ways. First, the classification cells rank galaxies by various physical parameters, thereby providing insight into galaxy evolution (section 2.3). Second, they partially isolate different physical processes. The development of galaxy morphology consists of revisions and refinements intended to improve these two types of result. The ultimate aim is to construct a physical taxonomy, i.e., a categorization by formation mechanism, physical structure and content. One approach to the eventual construction of such a physical morphology is discussed in section 2.4.
Morphological studies of the patterns of galaxy behavior (e.g., section 2.5) lead to results of two distinct kinds. Certain specialized questions can be answered definitely. One example is the study of galaxy shapes, including the deduction that many unbarred disks are globally oval. However, morphology is more generally a "soft" science, which is best viewed as preparation for more quantitative work. Its most important use may be to provide a list of specific questions which provide direction for this work. It is difficult to overestimate the importance of asking the proper questions, which routinely (if non-trivially) lead to concrete results even for recalcitrant problems. This section and section 5 illustrate the use of morphology both to reach specific conclusions and to establish questions for quantitative work.