Function or grammatical words

Function words (also called functors) are words that have little lexical meaning or have ambiguous meaning and express grammatical relationships among other words within a sentence, or specify the attitude or mood of the speaker. They signal the structural relationships that words have to one another and are the glue that holds sentences together. Thus they form important elements in the structures of sentences.
Words that are not function words are called content words (or open class words, lexical words, or autosemantic words) and include nouns, most verbs, adjectives, and most adverbs although some adverbs are function words (like then and why).
Function words might be prepositions, pronouns, auxiliary verbs, conjunctions, grammatical articles or particles, all of which belong to the group of closed-class words. Interjections are sometimes considered function words but they belong to the group of open-class words. Function words might or might not be inflected or might have affixes. Function words belong to the closed class of words in grammar because it is very uncommon to have new function words created in the course of speech.