Nouns can be categorized into common or proper, countable or uncountable, and concrete or abstract. Common nouns refer generally while proper nouns specifically identify a single person, place or thing. Countable nouns can be pluralized while uncountable nouns cannot be counted. Concrete nouns refer to tangible things and abstract nouns refer to ideas and qualities. Compound nouns are made of two or more words joined together and pronouns take the place of nouns in sentences.