Count nouns are words that refer to items that can be counted individually. They have both singular and plural forms, such as apple (one apple) and apples (more than one apple). Count nouns can be used with numbers and can take articles like "a" or "an" in the singular form. Examples of count nouns include dog, car, and book. In contrast to mass nouns, which refer to substances or concepts that cannot be counted individually (like water or information), count nouns can be easily quantified and categorized.