Here is a very basic introduction to manual exposure that I wrote up a while back:

Basic Technical Explanation

ISO sensitivity: the ISO [speed] is the sensitivity of the chip (or film) to light.
The higher the ISO number, the more light the chip (or film) can soak up;
Resulting in better lighting in low-light conditions. However, the higher
the ISO speed (number), the higher the noise (grain) levels. You generally
want to stay below ISO 1600 (depending on the quality of
the camera you are using). Generally, you want to keep your ISO speed
as low as you can (unless you are purposefully going for a grainy effect)

Shutter speed: The shutter is a little door on the inside of the camera that opens when you
take a picture. When the shutter opens, it allows light to shine on the chip
(or film) and write or burn the image to the chip or film. the longer the
shutter is open, the more light is let in. shutter speeds are displayed as:
1/125 (being a one hundred and twenty fifth of a second) or a single
number such as 10 (being a ten second shutter). As a general rule, you want
your shutter speed to match your focal length. your focal length is the zoom
that your lens is at. your focal length is measured is millimeters. the smaller
the number, the shorter the zoom. a 15mm lens is very wide! its side wide
that the sides of the image are distorted! a 300mm lens would be a nice
telephoto lens. So if the focal length you are at is 55mm, you would want
you minimum shutter speed to be about 1/60sec. and for say, a focal length
of 135mm, you would want to keep you shutter speed at or above 1/140sec.
Of course there are exceptions. I’ve taken sharp pictures at 1/20sec. at a
focal length of 200mm! If your shutter speed is too low, a blurry image will
result. If your shutter speed is too high, there will not be enough light let in,
resulting in a dark image.

Aperture: The aperture is a little circular door that is housed in the lens. By adjusting the
size of the aperture, you are also adjusting the depth of field. Depth of field is
basically the amount of blur behind the subject (the object that the camera is
focused on). As with the shutter speed, the wider the aperture is open, the more
light is let in. but when the aperture is wide open, the objects behind the subject
become out of focus much faster than when the aperture opening is small. But
when the opening is small, (duh) less light is let in. Aperture is displayed on the
camera in f-stops. A large aperture opening would be displayed as f2.8 (as you
will notice, with aperture setting, the lower the number, the bigger the opening
and vice versa), and a small opening would be displayed as f23. A small
opening (such as f32) is good for landscape photographs (because of the very
long depth of field). A large opening (such as f2. is good for macro photos.


White balance: the white balance is a color adjustment for different light settings. For
example, when you are shooting photos during a sunset, you would
probably want to use the daylight setting. The different white balance
settings are pretty self explanatory. They are usually displayed as pictures
of the different light scenarios (such as a pictures of clouds for a cloudy
day, or a fluorescent bulb if you are in a room with fluorescent lighting).
There is also a white balance for when you are using flash, and an auto
white balance for when none others seem quite right.