The case of a letter refers to its representation.
The most known case are:
- upper case (ie UPPERCASE, LOWERCASE)
- and lower case (ie lowercase, uppercase)
but there is more:
- Camel Case: UserCount used in java code for instance
- Snake Case: USER_COUNT used in SQL for instance
They refer to the physical cases where printers kept their letters
Where you can see a physical representation of letter frequency.