A fundamental unit with which morphology is concerned is the word and its constituents. A word is a symbolic unit which is a combination of meaning and sound. Words may be simple, composite, or compound.
A simple word is composed of only one lexical structure. The word bird, for example, consists of one lexical constituent. It is the minimum free form which can stand by itself and act as a meaningful utterance.
A composite word, also known as complex is composed of two or more substructures, The word careful, for example, is a derivation from the lexical substructure care and ful.
A compound word is composed of two substructures. For example, the word bookstore is the combination of the lexical substructure book and store.
Simple, composite and compound words are subsumed under the general term morphological expression.