Directories Directories are used to hold ordinary and special files. Directories are similar to folders in Mac OS or Windows.
Invisible Files An invisible file is one whose first character is a dot or period (.). Many programs (including the shell) use such files to store configuration information. Invisible files are also referred to as hidden files.
Option An option is an argument that starts with the hyphen or dash character, '-'.
Ordinary File An ordinary file is a file that contains data, text, or program instructions. Almost all the files on a UNIX system are ordinary files.
Special Files Special files are mainly used to provide access to hardware such as hard drives, CD-ROM drives, modems, and Ethernet adapters. Some special files are similar to aliases or shortcuts and enable you to access a single file using different names.