This article is about the file-system concept, for a Helpdesk directory see List of Support Articles.
A directory (also known as a "folder" on some UNIX and other operating systems) is a special file in a file-system that can contain regular files or other directories. Normally users refer to directories as "folders" because that is what they are.
On UNIX file-systems a directory is a file (technically referred to as a "node") just like any other file and thus any file operation can be performed on a directory. This has certain disadvantages, for example a file cannot have the same name as a directory in the same directory as the file.
On UNIX directories are addressed through "paths" which are strings of directory names separated by virgules ("/"). All folders are located under a "root" folder which is simply referred to as "/", in this manner a UNIX file-system resembles an upside-down tree and is often referred to as such.
Thus, one ascends and descends the file-system tree as one gets closer to root and deeper into subdirectories, respectively. Most graphical file managers also use this paradigm.
A sample path is /home/nick/somefolder/somefile. The initial "/" refers to the file-system root. (If it were not present this would be a "relative path" which is a path relative to the unspecified current directory.) Each subsequent path is a separator indicating one more level "down" in the directory tree. Thus "home" is a folder in "/" "nick" a folder in "home" and "somefolder" a folder in "nick". The only way we know that "somefile" is a folder in "somefolder" and not another directory is the name, a path can go on indefinitely.
In this diagram the root is to the left and thus left equals "up" and right equals "down".