dir (directory) is a command used for file and directory listing, specifically in the command line interface (CLI) of the operating systems CP/M, DOS, OS/2, Singularity, Microsoft Windows and in the DCL command line interface used on VMS, RT-11 and RSX-11. The command is also supplied with OS/8 as a CUSP (Commonly-Used System Program).
The following example demonstrates the output of the dir command on Windows 7, without arguments:
C:\Users\Public\Pictures\Sample Pictures>dir Volume in drive C is System Volume Serial Number is C8D0-76E2 Directory of C:\Users\Public\Pictures\Sample Pictures 14-07-2009 06:41 <DIR> . 14-07-2009 06:41 <DIR> .. 14-07-2009 06:52 879,394 Chrysanthemum.jpg 14-07-2009 06:52 845,941 Desert.jpg 14-07-2009 06:52 595,284 Hydrangeas.jpg 14-07-2009 06:52 775,702 Jellyfish.jpg 14-07-2009 06:52 780,831 Koala.jpg 14-07-2009 06:52 561,276 Lighthouse.jpg 14-07-2009 06:52 777,835 Penguins.jpg 14-07-2009 06:52 620,888 Tulips.jpg 8 File(s) 5,837,151 bytes 2 Dir(s) 57,925,980,160 bytes free C:\Users\Public\Pictures\Sample Pictures>dir /w Volume in drive C is System Volume Serial Number is C8D0-76E2 Directory of C:\Users\Public\Pictures\Sample Pictures [.] [..] Chrysanthemum.jpg Desert.jpg Hydrangeas.jpg Jellyfish.jpg Koala.jpg Lighthouse.jpg Penguins.jpg Tulips.jpg 8 File(s) 5,837,151 bytes 2 Dir(s) 57,925,992,448 bytes free C:\Users\Public\Pictures\Sample Pictures>
C:\>HELP DIR DIR [drive:][path][filename] [/A[[:]attributes]] [/B] [/C] [/D] [/L] [/N] [/O[[:]sortorder]] [/P] [/Q] [/R] [/S] [/T[[:]timefield]] [/W] [/X] [/4] [drive:][path][filename] Specifies drive, directory, and/or files to list. /A Displays files with specified attributes. attributes D Directories R Read-only files H Hidden files A Files ready for archiving S System files I Not content indexed files L Reparse Points - Prefix meaning not /B Uses bare format (no heading information or summary). /C Display the thousand separator in file sizes. This is the default. Use /-C to disable display of separator. /D Same as wide but files are list sorted by column. /L Uses lowercase. /N New long list format where filenames are on the far right. /O List by files in sorted order. sortorder N By name (alphabetic) S By size (smallest first) E By extension (alphabetic) D By date/time (oldest first) G Group directories first - Prefix to reverse order /P Pauses after each screenful of information. /Q Display the owner of the file. /R Display alternate data streams of the file. /S Displays files in specified directory and all subdirectories. /T Controls which time field displayed or used for sorting timefield. C Creation A Last Access W Last Written /W Uses wide list format. /X This displays the short names generated for non-8dot3 file names. The format is that of /N with the name inserted before the long name. If no short name is present, blanks are displayed in its place. /4 Displays four-digit years.
Switches may be present in the DIRCMD environment variable. Override preset switches by prefixing any switch with - (hyphen) -- for example, /-W. 
dir is not a Unix command, Unix has the analogous
ls command instead. The Linux operating system, however, has a
dir command that "is equivalent to
ls -C -b; that is, by default files are listed in columns, sorted vertically, and special characters are represented by backslash escape sequences," as the documentation says.
- Windows 7
- dir invocation (GNU coreutils) at www.gnu.org