Capital letters are the larger letters in the alphabet, used to signify the beginning of sentences and proper nouns. For example, the first letter of a sentence, like in the word Apple, is capitalized. They help readers identify important words and understand the structure of written language. In English, capital letters are also used for titles, names of people, places, and organizations, such as John, Paris, and United Nations. Additionally, they can emphasize certain words in writing. Understanding when to use capital letters is essential for clear and effective communication.