There are important differences between what we call grammar and what we call the parts of speech.

Grammar is the set of rules that govern how the things referred to in a sentence relate to each other. Grammar speaks of subjects, predicates, and objects. The parts of speech are things like nouns, verbs, adjectives, and adverbs.

The parts of speech are a relatively small number of categories that describe the nature of the concepts referred to by words.  (Grammar is concerned with the way that words relate to each other.)

Consider this sentence from Shakespeare: “Suspicion always haunts the guilty mind.” In terms of grammar, suspicion is the subject that performs an action, haunts is the action being performed, and mind the object of that action. In terms of the parts of speech, however, suspicion is a noun, always an adverb, haunts a verb, the an article, guilty an adjective, and mind a noun.

The exact number and names of the parts of speech are not agreed upon by grammarians, but most include a list that looks like this: nouns, pronouns, verbs, adjectives, adverbs, prepositions, conjunctions, articles and interjections.

The same word can be used in different ways, which means that one word can belong to multiple parts of speech, depending on its use. With language, as with concepts (e.g., truth, virtue, beauty), meaning is not absolute and universal; meaning is contingent on use. For example, walk can be either a noun or a verb: I went for a walk, or, I will walk home.  Additionally, walking can be either a verb or a noun: I am walking home, or, Walking is a real joy. You must examine how a word is used in a sentence to know with certainty which part of speech it belongs to.

Noun: A word that names a person, place, thing, or idea (e.g. queen, Queen Elizabeth I, country, England, body, love, milk, philosophy, stars, quotes, audience, children, doctor, Kansas, car-ride, concept, igloo, wealth, Australia).

• I know I have the body of a weak and feeble woman, but I have the heart and stomach of a king, and of a king of England too. – Elizabeth I
• Knowledge is power. – Bacon

Pronoun: A word that takes the place of a noun (e.g. I, me, we, us, you, she, her, he, him, it, they, them, that, which, who, whom, whose, whichever, this, that, these, those, both, few, all, himself, yourself, most, mine, ours, someone, their).

• It is myself that I portray…. I am myself the matter of my book. – Montaigne
• You are a king by your own fireside, as much as any monarch in his throne. – Cervantes

Verb: A word that shows action, occurrence, or a state of being (e.g. is, am, are, was, were, love, writes, kicked, to taste, was undergoing, will be, will be dining, might have thought).

• I think, therefore I am. – Descartes
• If I have seen further it is by standing on the shoulders of giants. – Newton

Adjective: A word that modifies a noun or pronoun (e.g. English, happy, old-fashioned, blue, famous, low, colossal, ancient, delicious, few, this, hot, hotter, hottest, unique, five).

• In such condition there is … no arts; no letters; no society; and which is worst of all, continual fear and danger of violent death; and the life of man, solitary, poor, nasty, brutish, and short. – Hobbes
• I cannot praise a fugitive and cloistered virtue, unexercised and unbreathed, that never sallies out and sees her adversary. – Milton

Adverb: A word that modifies a verb, an adjective, or another adverb (e.g. dearly, awkwardly, sometimes, soon, rather, well).

• To speak, and to speak well, are two things. – Jonson
• Melancholy and despair, though often, do not always, concur. – Burton

Preposition: A transitional word that leads into a modifying phrase (e.g. above, despite, for, in, inside, like, as, of, to, with).

• In the country of the blind the one-eyed man is king. – Erasmus
• All places, all airs make unto me one country; I am in England, everywhere, and under any meridian. – Browne

Conjunction: A linking word that connects words or groups of words through coordination (e.g. for, and, nor, but, or, yet, so) or subordination (e.g. if, because, when, since, although, unless, as if, insofar as, in order that, to the extent that).

• Since love and fear can hardly exist together, if we must choose between them, it is far safer to be feared than loved. – Machiavelli
• Man is neither angel nor beast; and the misfortune is that he who would act the angel acts the beast. – Pascal

Article: A word used with a noun or noun phrase that expresses the distinctiveness of the noun. That is, articles tell you how specific or general the noun. In English there are definite articles (the), indefinite articles (a/an), partitive articles (some), and negative articles (no).

• Was this the face that launched a thousand ships? – Marlowe
• No man is an island, entire of itself; every man is a piece of the continent, a part of the main. – Donne

In informal speech, there are also interjections, or words that convey emotion (e.g. ahem, darn, oops).