Uppercase letters, also known as capital letters, are the larger of the two types of letters in the English alphabet. They are used for the beginning of sentences, proper nouns, and abbreviations. Uppercase letters are also used to signal to the reader that something is important or significant. Most uppercase letters look similar to their lowercase counterparts, but they are larger in size.

On the other hand, lowercase letters are used for all the remaining letters in sentences and following the first letter of proper nouns. They are used for common nouns and for every letter after the first letter of proper nouns. Lowercase letters are smaller in size than uppercase letters.

In summary, uppercase letters are used for specific circumstances such as the beginning of sentences, proper nouns, and abbreviations, while lowercase letters are used for all other letters in sentences and following the first letter of proper nouns.