Have you ever taken a photo of a bright scene only to have it turn out under exposed (darker than it looked)? Or perhaps a night scene that ended up far too bright? The reason for the under/over exposure is your camera is trying to adjust so it exposes for an average of middle grey (or 18% Grey).
You can correct these problems with a Kodak Grey(Gray) Card. By metering off a grey card that is already set for 18% you should get correct exposure for your scene, at least that is the theory and for the most part it works really well.
But why grey? Well your camera's light meter can't see in colour. It only sees reflected light in shades of grey. Of the entire range of reflected light that your camera meter sees, 18% grey is the middle, hence the term "middle grey". All built in camera light meters work in this way. Two stops underexposed from middle grey is close to, but not quite, pure black. Two stops overexposed from middle grey is close to, but not quite, pure white.
To meter off of a grey card, you need to hold the card in the light that is falling on your subject so that card is angled slightly toward the primary light source (this is usually the Sun). You need to fill the frame with the grey card but it doesn't matter why type of metering you have your camera set to. If you have spot metering set, you don't need to be so concerned with filling the frame so long as your spot meter reads the grey card.
One of the additional benefits of using a grey card is that it also corrects the colour balance of your scene. The back of the grey card is also white, which can be used to capture a correct white balanced image. This can be used either in camera to set a custom white balance, or later in your photo-editing package. You will need to read your cameras instruction manual for details on how to do this. The white side of the card is also an excellent tool to reflect added light onto your subject to lessen harsh shadows... if you're working with close up subjects.
Now lets extrapolate this a little further.
Once you've become familiar with concept of using a grey card you'll find there is another excellent source for metering a scene. A bitumen surface. Bitumen roads are usually a shade of grey, and you may not always have your grey card with you. Next time you’re standing on a road, hold your grey card against the surface and check the difference. An older surface will be close to your grey card, a newer or wet surface will be darker. Once you can recognize the difference, you can adjust your meter reading to cope with the change in shade of grey.
Another source of approximate middle grey in nature the blue sky. If you find yourself out and about and you don't have your grey card with you and you're not standing near a bitumen road, point your camera at the blue sky and meter from the area that is darkest blue. I use this method a great deal and it works amazingly well while the sky is a recognizable blue.
But what if you don't have a blue sky, you don't have a grey card and your not near a bitumen road? Try green grass! This is usually a little darker than middle grey so you will need to underexpose by around half a stop.
And finally, if your camera allows for it, always shoot in RAW. This will give you the best chance of adjusting your image later down the track if you completely fouled things up.
Now you have no excuse for not capturing correctly exposed images every time regardless of your situation. Happy snapping.