Digital cameras use sophisticated exposure systems with a choice of metering patterns to suit different lighting situations. The systems work on the assumption that the area of the scene being metered is a mid-tone, or 18% gray to be precise; the average if all dark, light and mid-tones were mixed together. It's the basis of all metering patterns and works surprisingly well, but can render incorrect exposures when the overall scene or subject is considerably lighter or darker than 18% gray. For example, very dark areas can fool the metering system into overexposure, as the light meter will take a reading that renders it as a mid-tone.
As a camera is trying to render an image 'gray', it's your job to ensure you compensate to keep the tones true to life. You can do this by either using one of your camera's exposure override facilities, such as exposure compensation, the AE-Lock button or by metering from an area of the scene that has a mid-tone. And that's where our gray card comes in. Using it is very simple. The key thing to remember is that you need to place the gray card in similar lighting to your subject, for instance, don't place it in a shaded area if your subject is bathed in sunlight. Also, make sure that the card fills the metering area -- we would recommend you use spot or partial metering as the card won't need to fill the entire image area -- but any is suitable.
You can either lock the exposure using your camera's AE-Lock facility or note the aperture and shutter speed, then switch to manual mode and dial in these settings. This latter method isn't suitable on days where lighting is variable. The card has AF reference lines to help your camera's autofocus lock on to it. However, you don't necessarily need it to be in focus to work correctly. The gray card (as well as the white card) can also be used to take a custom White Balance reading from, too.