A case converter is a program or function that changes the case of the letters in a text string. The two main cases used in writing are upper case and lower case. An upper case converter changes all lower case letters to upper case, while a lower case converter changes all upper case letters to lower case. Case converters are commonly used for text processing tasks, such as converting data to a standard format, preparing text for searching, or changing the case of filenames. The conversion can be done on a word-by-word basis or the entire text string.