Morphemes are the smallest units of meaning in a language. There are two types of morphemes: bound morphemes, like prefixes and suffixes, which cannot stand alone and are attached to other morphemes; and free morphemes, like words, which can function independently. Examples of bound morphemes include "de", "dis", and "extra", while examples of free morphemes are words like "dog", "town", and "city". Infixes are another type of bound morpheme that are inserted within words, though they are rare in English.