Uppercase letters, also known as capital letters, are one of the two forms that the 26 letters in the English alphabet can take. They are used for the beginning of sentences, proper nouns, and abbreviations. Uppercase letters are larger than their lowercase counterparts, and most of them look similar to their lowercase partners, although some take slightly different forms. Lowercase letters are used for all the remaining letters in sentences and following the first letter of proper nouns.

Examples of uppercase letters are A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, and Z. They are used to signal to the reader that something is important or significant. Uppercase letters are also used in titles, which are considered proper nouns and require capitalization.

In summary, uppercase letters are one of the two forms that the 26 letters in the English alphabet can take, and they are used for the beginning of sentences, proper nouns, abbreviations, and titles.