Linguists divide words into two classes: content words and function words.
Content words give us the main information by carrying the meaning (content). They include nouns, adjectives, verbs and most adverbs.
Content words are open-class words meaning new content words can be added to a language relatively easily. This can be through compounding (joining two or more words together to create a new one), derivation (usually adding an affix to the end of an existing word to create a new one), inflection (modifying an existing word to express a different grammatical category), coining (either a new usage of an existing word or a completely new word), and borrowing (taking a word from another language).
Function words are the words that make sentences grammatically correct and have little meaning on their own. They include conjunctions, prepositions, pronouns, auxiliary verbs and determiners. If we leave out function words or use them incorrectly, our listener or reader would still probably understand our meaning.
Function words are closed-class words meaning they’re very resistant to change; it’s rare for new words to be accepted into this word class. Compared to content words, there are always relatively few function words in a language.