Text or typed input 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. On modern systems, passwords are case-sensitive, and usernames are usually case-sensitive as well.
If you cannot log into an account, make sure caps lock isn't enabled and that the correct case for each letter in your username and password is used.
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 works for the ls command in Linux. Typing LS would generate an error, for instance.