Short answer: They are 2 different operating systems that use different Syntax and Commands
Long Answer:
Back in the days before Microsoft Windows dominated the PC market, operating system were controlled by commands. Prior to MS-Windows, PC users were required to learn these commands in order to perform routine tasks. During the 1980s, Microsoft DOS dominated the PC market while the early UNIX command systems were used on larger multi-processing servers.
The main difference between UNIX and DOS is that DOS was originally designed for single-user systems, while UNIX was designed for systems with many users.
Because UNIX and MS-DOS were developed at the same time they share some common syntax, and the UNIX admin will be happy to find many common commands and concepts. The list below shows some of the commonality between UNIX and MS-DOS commands:
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Changing directories: both DOS and UNIX utilize the cd commands for this purpose, but UNIX has the "cd " command that allows you to flip-flop between your last two directories.
Display file contents: UNIX has the "cat" command" and DOS uses the "type" command.
Setting file permissions: DOS uses the "attrib" command while UNIX uses the "chmod" command to change file permissions.
Copying files: DOS uses the "copy" command, while UNIX uses the "cp" command.
Comparing two files: UNIX uses the "diff" command and DOS has the "fc" command.
Printing files: UNIX uses the "lp" command while DOS uses the "print" command.
Moving files: DOS uses the "rename" command and UNIX uses the "mv" command.
Make a directory: UNIX uses the "mkdir" command while DOS has the "md" command.
Display directory contents: UNIX uses the "ls" command and DOS has the "dir" command.
Delete files: DOS uses the "del" command while UNIX has the "rm" command.
Display the OS version: DOS uses the "ver" command while UNIX has the "uname -a" command.
EDIT: There was some VERY outdated information in my post so I had to edit it...