Morphology is an area of study within grammar that is concerned with how words are composed. The basis of analysis is the morpheme, defined as the minimal unit of meaning. Morphemes may stand as separate words in some languages, including English, or they may occur as parts of words called inflections or affixes, such as English suffixes. A single morpheme that can stand on its own is called a free morpheme, and one that can occur only a part of a word is termed a bound morpheme. The most obvious example of a morpheme is the suffix "s" or one of its variants, which marks English plurals. It may take a variety of phonetic forms (s, z, es, en), which are known as allomorphs of the same morpheme. The word "students," therefore, is made up of two morphemes: "student" and "s," where "s" indicates that the word refers to more than one student. The singular form could have also been marked by a special morpheme, but morphemes for only one object are not used in English. Linguists assign a zero morpheme to such a situation. (English plurals sometimes take zero morphemes, e.g., fish.)

The presence of the plural morpheme has two important implications in English. First, it identifies a category of words called nouns by the principle that all nouns can assume a form that indicates plurality. This statement stands as the morphological definition of the noun as a part of speech and serves as a more accurate definition than the traditional one that a noun names something. Second, morphological pluralization divides nouns into two types, called noun classes, i.e., singulars and plurals. English noun classification can then be compared to that of another language that might use a more intricate numbering system that includes duals for things that occurred in pairs as well as singulars and plurals.

Languages differ in many areas of noun classification. Aside from number classes, many languages differentiate nouns on the basis of gender. French has two noun classes, masculine and feminine, which are often morphologically marked within the word and through article and adjectival agreement, as in the strings: "le professeur jeun" and "la professeuse jeune." (Note that the morphological markers here are the differences in pronunciation not those of spelling). German, which includes a neuter form, has a different morphological gender system from French.

A third noun-class system is observable among many African languages, especially those of the Bantu group. Here nouns contain morphemes as prefixes that mark many different aspects of the noun, sometimes as many as fourteen. Distinctions formed can indicate whether the object is animate or inanimate, plant or animal, or human or nonhuman. Abstract concepts may also be grouped in differing noun classes on the basis of common characteristics. Thus in the country of Uganda there is a major ethnic group whose members are called Baganda, live in the traditional kingdom of Buganda, and speak a language called Luganda. Similarly, Swahili, another Bantu language, should actually be called Kiswahili according to its noun-class prefixes. A Swahili speaker, on the other hand is a Waswahili.

Other parts of speech are also morphologically defined and divided into classes. Verb classes are usually the most complex systems within the morphology and can mark time divisions, intentions, and states of being.