Upper and lower case
Upper case refers to letters which may also be known as capital letters or majuscules for example in the Roman alphabet: A, B, C, D..., and lower case to the minuscule letters: a, b, c... Some languages such as Arabic and Chinese do not make a distinction between upper and lower case letters in their writing systems - and indeed the distinction was not made in European languages until the 1300s where minuscule and majuscule letters did exist but were used indiscriminately depending on the source of the text.
In the days when printers prepared a page of print by picking out letters of metal type one by one and inserting them into a form ('hand setting'), they kept these letters in different compartments in boxes which could be closed and transported. These boxes were called cases. When they were in use, they were kept on two levels. The printer would stand one case higher than the other.
The capital or majuscule letters (A, B, C... X, Y, Z etc) were kept in the upper case, while the ordinary, 'small' or minuscule letters (a, b, c... x, y, z) were kept in the lower case.
That is why these are the terms used in technical descriptions of printing and other uses of the written language, for example in Microsoft Word and other computer programs. The terms upper and lower case are now the standard way to name these different sizes of letter, and, in proof-reading and other instructions to printers, their abbreviated forms u.c. and l.c..