All the iDevices use HFSX as their filesystem, a variant case of HFS+. It is possible to store two or more files with the same name within the same folder, but with a different case of each individual character (for example,
HFS Plus (or HFS+) is the filesystem developed by Apple to replace, from Mac OS 8.1, HFS as the default filesystem for Mac computers. In Apple's official documentation, it is called Mac OS Extended.
HFS+ is an improved version of HFS, which allows the user to support larger files (thanks to block addresses of 32 bits instead of 16 bits) and uses Unicode for the names of filesystem objects (files and folders), thus allowing up to 255 characters for each. Until Mac OS X Tiger, ...