Countable nouns are words that refer to items that can be counted individually. They 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 the 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 helps in using them correctly in sentences.