A countable noun is a type of noun that can be counted individually. These nouns have both singular and plural forms, meaning you can say one apple or two apples. Countable nouns can be used with numbers and can be preceded by articles like "a" or "an" in their singular form. Examples of countable nouns include book, car, and child. In contrast, uncountable nouns, such as water or information, cannot be counted individually and do not have a plural form. Understanding the difference between countable and uncountable nouns is essential for proper grammar usage.