Well, judging from his other posts, I am still not sure whether he is being serious in his questions and / or he is looking for shortcuts. I'll give him the benifit of the doubt at this point and see how things progress.

1. DOS is an Operating System (See 2). This was used during the 80's before windows came out for PC's Microsoft had a version, called MS-DOS and the other big contender at the time was DR-DOS from Digital Research. These two companies were at the forefront of what later became know as the DOS wars.
2. Operating System (OS). This is a series of softwares that interact with the hardware and non-OS related software. (My definition, not Websters) There are many OS's including:
MS-DOS
DR-DOS
Linux
Unix
Be
OS2
etc....

You can check out Google for more information but I would suggest that you take what you read with a grain of salt. Not everything you read on the Web is correct and you have to learn how to read from differnt sources and find the correct information. It sounds to me like you are very new to this so that is why I am suggesting that you read Google with care. Most of us who have been working with computers for years know this and we don't even think about it. I can create a web site that say that MS-DOS is an operating system for Sun workstations. That statment would be wrong but if I set the page up in a certain way, and you were to do your Google search a certain way, it would be possible for you to come across my web site and read this incorrect information. If my site was the only one you checked out, you might be lead to believe that my info was correct and in fact, it and you would be wrong.

Sorry about the extened stuff about Google. I just want to make sure he gets on the right track.