A count noun is a type of noun that can be counted and has both singular and plural forms. For example, the word apple can be used as "one apple" (singular) or "two apples" (plural). Count nouns can be used with numbers and can take articles like "a" or "an" in the singular form. Count nouns are different from mass nouns, which cannot be counted individually and do not have a plural form. For instance, water is a mass noun because you cannot say "two waters" in the same way you would say "two apples." Understanding count nouns helps in proper grammar and sentence structure.