MS DOS is an operating system that was in use on all Intel based computer before the invention of Windows 95. It is a disk based operating system (meaning you load it from a floppy disk) and it has no fancy graphic interface. No icons, no mouse pointer, no menus, etc. It is entirely text based.
You can see a stripped down version of MS-DOS on a Windows computer by going the "run" on the Start Menu and typing in "cmd". This will open a windows with nothing but text in it. You have to know the DOS commands (such as "dir", "cd", "ren", "cls", etc.) to make anything happen. MS-DOS has not been used or sold in years.
To put a copy of Windows 7 on a computer, the manufacturer has to purchase it from Microsoft. Microsoft does give the manufacturers a huge break on the price over what they charge you to buy it in the store. But it is not free. So some companies that sell bottom end laptops really cheap will shave a few dollars off the price by selling you the computer without Windows on it. Once you get the laptop, however, you will find that you can not do anything with it.
It will not surf the Internet. It will not play music. You can not watch videos on it. You can not play any games on it. About all DOS does is allow you to manage files and move directories.
You would end up having to either purchase a copy of Windows ($200) or install one of the free Windows like Linux Operating Systems.
But you will probably find that a laptop with nothing but MS-DOS on it will be useless to you.