A noun is a word that represents a person, place, thing, or idea. Nouns can be categorized into different types, such as common nouns, which refer to general items like dog or city, and proper nouns, which name specific entities like John or Paris. Nouns can also be singular or plural, indicating whether they refer to one item or more than one. For example, the singular noun apple becomes the plural noun apples when referring to multiple items. Understanding nouns is essential for constructing clear and meaningful sentences.