Case-sensitive systems differentiate between uppercase and lowercase. “ABC” and “abc” are not the same thing on a case sensitive system. In web design it’s important to note that hosting servers running MS Windows are not case-sensitive (contactus.htm and ContactUs.htm are treated as the same file) while servers running Linux are case-sensitive (if the file is saved as contactus.htm, a link to ContactUs.htm would produce an error.) This is something to watch out for when changing from one operating system to another.