A morpheme is the smallest grammatical unit in a language.
Words is the smallest meaningful unit of a language.
A morpheme is not identical to a word, and the principal difference between the two is that a morpheme may or may not stand alone, whereas a word, by definition, is freestanding. When it stands by itself, it is considered a root because it has a meaning of its own (e.g. the morpheme cat) and when it depends on another morpheme to express an idea, it is an affix because it has a grammatical function (e.g. the –s in cats to specify that it is plural).[1] Every word comprises one or more morphemes. The more combinations a morpheme is found in, the more productive it is said to be.
Words can be put together to build larger elements of language, such as phrases (a red rock), clauses (I threw a rock), and sentences (He threw a rock too, but he missed).