Gray cards are designed to reflect 18% of the light as stated above and they are a good starting point for most exposures but are rarely "perfect" exposure. You can also use something like a tribalance card which has grey, white and black. You take the shot with the triblance in the picture and can later determine the white piont of the image as well as the dynamic range (to an extent). Alot of people also recommend the 12% grey card which is made by Lastolite. Don't get a cheap grey card, the grey fades over time.
Another important skill to develop is the ability to determine a 50% grey in a coloured scene, like a landscape for instance. You can practice by getting a reading off the grey card and then pointing the camera at something that gives you the same or similar reading, like concrete, darker grass patches, some types of bark etc. This will train your eye for exposure outdoors for when you don't have a grey card with you. I also take a couple of shots before hand and then check them in the LCD. Zoom in, have a good look. This way, no matter where you are, even with no grey card you should reach the best exposure values in less than half a minute.
This isn't the end of story however. How often do you see landscape shots where the sky is exposed well but the details in the shadows are lost or the details are there but the sky blown out? This is why a good landscape photographer will always shoot multiple exposures often in raw and then PP for an excellent outcome. Good luck.
|Valley by the light of a blue moon by cjf2|
from Down in the Valley
|Lake Erie Stone Pier by yobbyt|
from Dock or Pier