What is DOS?  DOS stands for Disk Operating System.  DOS was the operating system used by many PCs when computers first became available.  Following are some standard DOS commands.

cd A DOS command which will change the directory in which you are working dependant upon the commands following the change directory command.
copy A DOS command which will copy a file.  For example "copy command.com a:" will copy the "command.com" file to the A: drive.
del A DOS command which will delete a file.  For example, "del example.doc" will delete the file called "example.doc".
deltree A DOS command which will delete all files, folders and subfolders. 
dir A DOS command which will list all files and folders located in a particular directory.
dir /p A DOS command which will list all files and folders located in a particular directory broken up by page breaks.
edit A DOS command which will allow you to edit a file.
exit A DOS command which will allow you to exit DOS.  This command is only good if you have entered DOS through a Windows command.
fdisk The program provided by Microsoft through DOS to create partitions on hard drives for systems using non-NT versions of Windows.  Fdisking will allow a 20 GB hard drive to be broken up into smaller, partitioned drives so that older Operating Systems can use the drives.
format A hard drive, floppy disk and other items must be formatted before they can be used.  Some formats completely erase all the information from a drive while others setup the framework for information to be written to the drive. Formatting can be done through Windows or DOS.
md A DOS command which will make a directory.  For example, "md test" will create a folder called "test" which will reside in the directory you are currently in.
mem A DOS command which will list the amount of memory on a computer.
rd A DOS command which will remove a directory.  For example, "rd test" will delete the folder called "test".
ren A DOS command which will rename a folder or file.  For example "ren example.doc example1.doc" will rename the file called "example.doc" to a new file called "example1.doc"
sys A DOS command which will load system files onto a hard drive or floppy drive. The system files "command.com", "io.sys" and "msdos.sys".
type A DOS command which will allow you to see the ASCII / text of a file.