The document discusses key concepts in morphology including words, morphemes, and affixes. It defines a word as a sound or group of sounds that expresses an idea, and notes that orthographically a word is a group of letters separated by spaces. It distinguishes between word forms, which are the concrete representations of lexemes, and lexemes, which are the abstract representations. The document also defines bound and free morphemes, noting that bound morphemes like prefixes and suffixes must attach to other morphemes while free morphemes can stand alone. It provides examples of roots, stems, bases, prefixes, suffixes, and infixes.