Nouns are words that name people, places, things, or ideas. They are essential in constructing sentences as they serve as the subject or object. Common nouns refer to general items (like dog, city) while proper nouns refer to specific ones (like Max, Paris). Nouns could also be concrete (table, book) or abstract (love, freedom). In a sentence, they can be used as the subject (The dog barks), object (She loves music), or complement (She is a doctor).