Common nouns are general names for people, places, things, or ideas. They do not refer to a specific individual or entity but rather to a class of items. For example, words like dog, city, and happiness are all common nouns. They can be singular or plural and are not capitalized unless they begin a sentence. Common nouns can be categorized into different types, such as concrete nouns, which refer to physical objects like apple or car, and abstract nouns, which represent ideas or concepts like freedom or love. Understanding common nouns helps in identifying and using language effectively in everyday communication.