Sentences are made up of different kinds of words, and it makes it easier to refer to them if we give them names depending on what they do. Many of us may have been taught at school that words are divided into groups known as parts of speech. That is an incomplete description and is no longer much used. Linguists speak instead of word classes. These can be listed as nouns, lexical verbs, adjectives and adverbs, on the one hand, and determiners, pronouns, auxiliary verbs, adverbial particles, prepositions and coordinating and subordinating conjunctions on the other. Those in the first group are called lexical words. They carry the meaning of a sentence. Those in the second group are called function words and, among other things, they help relate one lexical word to another. Function words are members of a closed class of words, that is, they only rarely admit strangers into their ranks. In this they contrast with lexical words which belong to an open class. Lexical words can, and frequently do, allow others to join them. As our needs change, so does the language, and we create new lexical words as required.
The difference is perhaps clearer in an example. Let’s take that last sentence. If we leave out the three function words, the, in and an, we’re left with difference is perhaps clearer example. While that is not what we would normally say, there is enough for us to get a good idea of the meaning. When we put back the, in and an, little is added to the meaning, but we can understand the sentence more readily. The three function words have little meaning in themselves, but they give the sentence a structure which our minds are better able to process.