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[Japo]

The main reference on this is the comments (lines starting with a '#') in the default configuration file accessible from the Windows start menu. The following is an overview of the most useful options (it's not common having to tweak the rest, but feel free to ask about them) along with some tips. I recommend everybody to take a look at the "fullresolution" section.
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[sdl]
fullscreen

Self-explanatory. It can be set to "true" (to start DOSBox in full screen) or "false" (in a window), like many other on/off options. Also while DOSBox is running we can toggle between the two settings with Alt+Enter.
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[sdl]
fullresolution

The resolution which DOSBox will try to set your screen to when working fullscreen. Its values can be "original"--the game's own native resolution--or any particular setting like for example "640x480", "1920x1080", etcetera. Keep in mind that if you set a value that your hardware doesn't support (as may be the case of "original"), in the best case it will fall back to a close supported value, and the image will be surrounded by an empty black frame.

I recommend setting this to the highest supported resolution that has the same aspect ratio as your monitor. In the case of flat monitors (non-CRT), this should be the "native" resolution (as long as its pixels are square, which I believe is always the case). To know what resolutions your hardware supports (specially in the case of CRT monitors that may support any number or resolutions with non square pixels--with a different aspect ratio from the monitor itself), check the Windows desktop properties. The aspect ratio of a resolution is the ratio between width and height in pixels, for example the aspect ratio of 1024x768 is 1024/768=1.33 (=4/3, usually referred to as 4:3). Likewise the aspect ratio of a monitor is the ratio between its width and height in centimeters or inches.

For this to work fine you must also set "output" to anything but the default "surface", and preferably "aspect=true". (See below for these other settings.)

The desired effect is to display the game, at its originally intended 4:3 aspect ratio, making the most of the area of the monitor. If yours is an old 4:3 monitor, the game image should fill it completely.

However if your monitor is a "wide screen" you SHOULD see black stripes left and right. This is is NOT BAD, this is GOOD--as long as there are no similar black stripes at the top and bottom. If the image of a DOS game--intended for 4:3 aspect ratio--were to fill a wider display (like a 16:9), it would be distorted, and you wouldn't be really getting more of the area of your display, contrary to what you might thing at first, and even though many people actually try to remove those stripes when they see them. Click here to see an example of distortion and why it's undesirable.

However you should never see black stripes all around, at the top and bottom and left and right at the same time, as long as you use the combination "aspect=true" and "output=ddraw", and set "fullresolution" to a value that your hardware supports and has the same aspect ratio as your monitor.
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[sdl]
windowresolution

The size of the window that DOSBox will be running in when not in fullscreen, in a window. It can also be set to "original" or to a fixed value. You may want to choose a size as big as possible within the resolution Windows is running, just leaving some height for the title bar of the DOSBox window and the Windows task bar.
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[sdl]
output

The default "surface" doesn't support scaling, set to "ddraw" instead. See "fullresolution" above.
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[sdl]
sensitivity

The higher this value, the faster the mouse pointer will move inside DOSBox, and the lower the slower. Thus we can adjust the exact desired speed for each game.
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[sdl]
mapperfile

By pressing Ctrl+F1 in DOSBox (by default, can be also re-mapped) we can re-map the real to virtual keyboard as well as joysticks, so that F11 can be fullscreen instead of or in addition to Alt+Enter, A is B or B is A, some keys emulate a joystick for the game, or the buttons and directions in our real joystick map to keys for the game, etc.

This configuration is saved in a file, here in this line we can specify what file. So we can use different files for different games, if we use different configuration files for them.
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[dosbox]
captures

Folder where our screenshots and clips will be stored when captured.
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[dosbox]
memsize

Size of RAM memory emulated. The default size is 16 MB which is more than enough for most DOS games. You're not recommended to change this unless necessary because many old games may crash if they find more than 16 MB.
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[render]
frameskip

If our hardware isn't fast enough for a game, and we tried everything else (the options in [cpu] above all), we can set this higher than "0" to relieve it of some work and get hopefully better performance but with choppier animations. (The value is the number of frames that DOSBox skips before drawing the next one: if it's "1" only half of the frames will be drawn, if "2" a third, if "3" a quarter, etcetera.)
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[render]
aspect

Enable ("true") or disable ("false") aspect correction. It's necessary to enabled this to get non square pixels (as DOS resolutions 320x200 and 640x400 were intended to be displayed on the same 4:3 monitors as the square-pixeled 640x480 resolution). See "fullresolution" above.
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[cpu]
core

Mode of virtual processor emulation. The "dynamic" mode has the advantage of making the most of our computer's power, so we can reach around twice as high cycle counts as in other modes. We'll have to use this dynamic mode for the latest DOS games that need a powerful processor.

The disadvantage is that some games may crash in this mode, but work fine in "normal" CPU mode; the good news is that most of these are old and don't need high cycles anyway.

We can leave it as default in "auto", that makes DOSBox use the normal mode for old real-mode games and the dynamic one for protected-mode games, that usually need much higher cycles. (Don't worry if you don't know what the real and protected modes of x86 processors are--neither do I.)
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[cpu]
cycles

Very important, this value's the speed of the virtual processor emulated by DOSBox for our DOS programs. As we said it's related to "core", since our computer will be able to emulate higher cycles in "core=dynamic" than in "core=normal". Increasing the cycles above this limit, which for each mode depends on the power of our real processor, won't achieve anything good.

This option can be set to a numerical value, or "auto". The "auto" setting tells DOSBox to use 3000 cycles for old real mode games, and to emulate the fastest processor possible for protected mode games.

(Maybe you find it strange that a DOS game that worked fine in an old 486 CPU now may run too slow with DOSBox in a much more powerful machine; but it's easy to understand if you consider that DOSBox's compatibility's so good because it emulates hardware--that is hardwired electronic circuits--through software--a program.)
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[cpu]
cycleup

Amount that cycles go up every time Ctrl+F12 is pressed while DOSBox is running. A value below 100 is understood as a percentage of the previous cycle count.
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[cpu]
cycledown

Amount that cycles go down every time Ctrl+F11 is pressed while DOSBox is running. The same about values below 100 applies here.
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[sblaster]

Under this heading, the settings for the virtual Sound Blaster sound card are found. It's best not to change them but leave as default, and remember the values when the games or their setup programs ask about them, as the basic tutorial explains: Sound Blaster 16 card, port/address 220, IRQ 7, and DMA 1.
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[dos]
ems

Enable ("true") or disable ("false") expanded memory (something used formerly in x86 processor's real mode). Some games need it, whereas other few won't work if this option is enabled; so we can leave it enabled as default and disable it for the necessary games.
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[dos]
keyboardlayout

This setting's a feature of the latest DOSBox versions, the old ones had the USA keyboard only. We can change the default value to the national code we prefer, for example "sp" for Spanish keyboard layout.

This makes typing easier for non Americans, but be warned that some games (I'm thinking of X-Wing for example) are designed to be played with an American keyboard, and if we change the layout we'll find the controlling keys (which can't often be customized) out of reach--and so to play these games it's better to leave the "us" option even if that's not our usual preference. An alternative, more complicated solution in these cases, could be to use a non-US keyboard but then re-map the keyboard.
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[autoexec]

Under this heading there are no specific options, but every line put here will be run as a command when starting DOSBox. It's the place to insert instructions for virtual drive mounting, among other things.