Exposure is the amount of light that enters the camera when you take a photograph. That concept isn’t too complicated although understanding exposure is one thing and using your digital SLR camera to control it is another. The problem is that you can become so overwhelmed by everything else going on — working the controls of your digital SLR, framing the subject, focusing, and so on — that exposure might be the last thing you think about. The good news is that you can quickly learn enough about exposure to integrate it easily into your photography and start taking better pictures. To create the correct manual exposure, you have to juggle these three elements:
Aperture: Controls the opening in the lens. Large apertures let in more light. Smaller apertures let in less light. Larger apertures shrink the depth of the area that’s in focus (the depth of field). Smaller apertures increase it.
Shutter speed: Determines how long the shutter (the element that blocks light from reaching the sensor) remains open and the sensor is exposed to light coming through the lens. Faster shutter speeds let in less light than slower shutter speeds. Faster shutter speeds prevent camera shake and motion blur.
ISO: Formerly a measure of film speed, ISO now indicates the sensor’s sensitivity to light. Lower numbers (for example, ISO 100) are best for bright conditions. Higher ISOs (for example, ISO 400 or 800) are often necessary when indoors. The side effect of raising the ISO is increased noise. Noise is usually bad, but sometimes noise has the appearance of film grain, which isn't always bad.
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Source:http://www.dummies.com/how-to/content/manually-controlling-exposure-on-your-digital-slr.html
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