Text or inputted text that is sensitive to capitalization of letters. For example, "Computer" and "computer" are two different words because the "C" is uppercase in the first example and lowercase in the second example. In many cases with computers or accounts that require a logon to access, the username and password is case-sensitive, which means the username and password must have the correct case.
If you are unable to log into a computer or account, make sure the caps lock key is not on and that you are using the correct case for each letter in your username and password.
Anything that is not case-sensitive means that any uppercase or lowercase character can be entered. For example, the Windows command line or MS-DOS is not case-sensitive, however, the Linux command line is case sensitive. In other words, you could type "cd", "CD", "Cd", or "cD" for the cd command in the Windows command line without error, but only "ls" will work for the ls command in Linux. Typing in "LS" would generate an error.