These days the term DOS is usually assumed to mean MS-DOS, or Microsoft Disk Operating System. Although there are other operating systems known as DOS, MS-DOS is used much more extensively than any others, although few people venture into the dreaded command prompt when they can avoid it. DOS is a dark and scary place to be for those raised on GUI-based operating systems (like Windows.) The user needs to know exactly what to type to make things happen - otherwise you will hear nothing but "Bad command or file name". You'd think Microsoft would at least include a Help command.
Surprise: There IS a help command, and gosh darn it's a beauty. On a computer running MS-DOS 5.x or 6.x, simply type HELP at the command prompt (to run the file C:\DOS\HELP.EXE or C:\DOS\HELP.COM, depending on your DOS version) Don't know your DOS version? Type VER. If it responds with some form of Microsoft Windows, this will be a little harder for you.
Windows 95 users can find HELP.COM in the \Tools\Oldmsdos\ folder on the Windows 95 CD. It can also be found on the Windows 98 CD, and I believe the Windows ME CD, in the \Other\Oldmsdos\ folder. Windows NT, 2000 and XP offer it as an internal command, i believe - meaning you can just type HELP.
If you're THAT serious about sitting down and learning DOS, there are many websites listing all DOS commands, hell, there are probably nodes lying around here too that can offer you a full list. Using the Help file is much easier though, because you can access it anytime you need it.
Whether or not you find this Help file, on your CD's or on the Web, the use of /? is usually the better option. All internal DOS commands, 99% of DOS programs, and several Windows programs, can be run as follows: /?
This will offer a quick rundown of the program's usage. Want to see this in action? Start a Command Prompt window right now, and tell it 'cd /?', then 'md /?', then 'rd /?', then 'del /?' (without quotes or commas.) Oh, and you can 'exit' when you've had enough DOS.
One warning: some programs ignore the /?, and will do whatever they normally do when they are run. Luckily, most DOS commands which can harm anything need to be run with parameters in order to modify/delete/format stuff. It is unlikely that you will ruin anything until you try this with every program on your hard drive. Just try to avoid experimenting with commands that sound dangerous.
Trust me. DOS is really easy to learn if you learn commands only when you need them. DOS is really hard to learn if you sit and study it as though it's the topic of tomorrow's report which you haven't quite started. Just learn these commands, and build from there:
Remember that commands and filenames can be UPPER, lower, or MiXeD case as you see fit. DOS won't care.
Summary: When in doubt, use HELP. It's guaranteed to not accidentally do stuff you'd rather not have done. It covers functions used only in your config.sys file, ones that can't be typed at the command prompt. You don't need to know the name of the command you're looking for, you can just look through until you see something that sounds kinda like what you're after. And it has an interface. Yes, a DOS program with an interface! (Well ok, not the NT or 2000 or XP one. But the rest do!)
When you can't be bothered negotiating through some interface, use /?. It gives you the goods with minimal fuss. Oh, and if a command gives you more info than will fit on one screen...you can follow it up with |more. Example: XCOPY /? |more
Remember to type HELP /? at least once!