ArcGIS stores its persistent information in Windows files. The Windows file system is organized into drives (once upon a time these were actual physical disk drives), which contain files and/or folders. (A folder is just a special kind of file in which Windows stashes information about other files or folders.) Together, drives, folders, and files implement a simple hierarchical information management system, which lets you group bags of information (files) into nested categories (folders).
The combination of a drive name, one or more folder names, and a file name is called a pathname. Windows pathnames are written as follows:
Any time you're working on Windows, there's a notion in effect called the current folder (AKA the working folder.) For a Windows application, the current folder is usually the folder containing the document that it was started up with.
A pathname that doesn't begin with a drive letter is called a relative pathname. A relative pathname is assumed to have the pathname of the current folder prepended to it.
\is assumed to have just the current drive name prepended to it.
The special folder name
.. (two periods) is often
used in relative pathnames to refer to the parent folder
of the current folder.
H:\arf.txtboth refer to the same file.
Windows also recognizes a network name
(of the form
\\server) that is sometimes used
instead of a drive letter.
Pathnames containing a network specification are called UNC pathnames. (UNC means Universal Naming Convention, which means that only Microsoft uses it...)
IMPORTANT: While most (but not all!) of Windows supports UNC pathnames, much of ArcGIS does not. They can trigger all kinds of misleadingly-reported errors. Don't use them!
When working with ArcGIS, it's essential that you follow these rules when creating new files or folders:
Breaking any of these rules will result in a file and/or folder name that may work just fine in Windows, but may be inaccessible to ArcGIS. When this happens, the ArcGIS error message won't give you the least clue what's going on...