In contrast to content words, function words (also called grammatical words) are words that have little lexical meaning or have ambiguous meaning, but instead serve to express grammatical relationships with other words within a sentence. Function words are closed-class words. Languages do not easily add new words to this set. They are always relatively few and resistant to change. They are lexically unproductive and are generally invariable in form.
of, at, in, without, between
he, they, anybody, it, one
the, a, that, my, more, much, either,
and, that, when, while, although, or
verbs be (is, am, are), have, got, do
no, not, nor, as