Letter case is the difference between uppercase/capital letters and lowercase letters for certain writing systems. The name comes from letters on a printing press, where uppercase letters were stored on top of the lowercase letters in a drawer. Their formal names are "majuscule" and "minuscule" in the same order. Writing systems that have letter case include the Latin alphabet, the Greek alphabet, and the Cyrillic alphabet.
When the first alphabets were made, they only had capital letters. Overtime, especially during the Middle Ages, lowercase letters were made to write more quickly. Other than Greek and Latin, European languages made no clear differences between capital and lowercase letters until the year 1300.
Different languages have different rules for when to capitalize or not to capitalize letters. For example, German always capitalizes nouns, while English only capitalizes proper nouns, so while German would write "the automobile" as "das Auto", English would not do the same. However, nearly all languages with letter case capitalize the first letter of the first word of ever sentence.