File names and extensions 
A filename is a special kind of string used to uniquely identify a file stored on the file system of a computer. Before the advent of 32-bit operating systems, file names were typically limited to short names (6 to 14 characters in size). Modern operating systems now typically allow much longer filenames (more than 250 characters per pathname element).
Windows, DOS, and OS/2 
In DOS, Windows, and OS/2, the root directory is "drive:\", for example, the root directory is usually "C:\". The directory separator is usually a "\", but the operating system also internally recognizes a "/". Physical and virtual drives are named by a drive letter, as opposed to being combined as one. This means that there is no "formal" root directory, but rather that there are independent root directories on each drive. However, it is possible to combine two drives into one virtual drive letter, by setting a hard drive into a RAID setting of 0.
Common Windows directory structure 
- Recycle folder (hidden)
- Boot folder (hidden, since Windows Vista)
- Documents and Settings
- User folders (up to Windows XP, legacy and hidden since Windows Vista)
- IIS folder (if installed)
- created by Windows Performance Information and Tools
- Program Data
- Program data (hidden, since Windows Vista)
- Program Files
- Program files
- Program Files (x86)
- Program files for x86 (32 bit) programs on a x64 Windows
- System recovery informations (hidden)
- System Volume Information
- Part of system restore (hidden)
- User folders (since Windows Vista)
- Windows folder
Unix and Unix-like operating systems use Filesystem Hierarchy Standard as the common form for their directory structures. All files and directories appear under the root directory "/", even if they are stored on different physical devices.