Case sensitive
Text or inputted text that is capitalization sensitive. In other words, "Computer" and "computer" are two different words because the "C" is in uppercase and lowercase. Many times a password or username is case sensitive. This means your username or password must have the proper case or you will be unable to login. Often users will experience the inability to log into a case sensitive login because their caps lock key is on causing the case sensitive password or username to fail.
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.
Also see: Case, Keyboard definitions, Lowercase, Uppercase