When discussing about words, we sometimes make an important distinction between two types of words: content words and function words (also referred to as open-class words and closed-class words, respectively).
Examples of content words include the English words uncle, manage, huge and rapidly. These words belong to the major parts of speech that consist of nouns, verbs, adjectives and adverbs and represent ideas, actions, objects and attributes. They are also called “open-ended” or “open-class” words because we can frequently add new words to this class of words. That is, we can create and add an infinite number of new words to these classes.
On the other hand, function words are those which do not have clear lexical meanings or obvious concepts related to them. They are lexically unproductive and are generally invariable in form. They belong to grammatical or function classes that consist of a small number of fixed items, such as articles, demonstratives, quantifiers, prepositions and conjunctions.
These function words denote grammatical relations and unlike content words, words have little or no semantic content. Examples of function words in English include articles (the, a), demonstratives (this, that), quantifiers (most, few, some, little), prepositions (up, from, to, with), and conjunctions (but, or, and, yet). To illustrate, consider the articles the and a/an. The essential feature of these articles is that they function grammatically to indicate whether a noun is definite or indefinite (the teacher or a teacher).