What are four categories of nouns?

Nouns can be categorized into four main types: common nouns, which refer to general items or concepts (e.g., "dog," "city"); proper nouns, which denote specific names (e.g., "London," "Sarah"); collective nouns, which represent groups of individuals or things (e.g., "flock," "team"); and abstract nouns, which refer to ideas or qualities that cannot be physically touched (e.g., "happiness," "freedom"). Each category serves a distinct function in language.