The document discusses the concept of morphemes, defined as the smallest units of meaning in language, and categorizes them into two main types: free morphemes, which can stand alone, and bound morphemes, which must attach to other morphemes. It further divides free morphemes into lexical (nouns, adjectives, verbs) and functional (conjunctions, prepositions, articles), while bound morphemes are classified into derivational (which change word class) and inflectional (which indicate grammatical function). Additionally, the document explains the concepts of morphs, which are the physical forms of morphemes, and allomorphs, which are the variations in pronunciation of a morpheme.