Upper case letters are the larger version of letters known as capitals while lower case letters are smaller versions. The lower case letters developed from the Middle Ages because they were faster to write and easier to read. Each letter in the Latin alphabet has an upper and a lower case.
There are four general standard rules for using upper case letters in Modern English. They are used when they are the first letter in a sentence and when they are the first letter or a proper noun, such as a name. All letters are written in upper case in initials and acronyms. In all other instances, lower case letters are typically used.