Case-sensitive

one of two options for taking into account all differences of letter case when comparing or searching for texts (example: case sensitive comparison of "DOG" with "dOG" returns "different")

If something is case-sensitive uppercase and lowercase letters are treated as two different things. This has become common since computerisation. But it is not always true. Passwords are generally case-sensitive, but email addresses are not. urls are usually case-senstive but search engines may not be.