I'll jump in, yes, a gray card is a gray card! =D I have the basic kind, just cardboard. There are other fancy dancy ones that are more "travel" friendly (fit in your camera bag), lens caps, etc. that all are about custom white balancing.
Also, the original purpose of a gray card concerned metering, and you can still use it that way. Basically you get it w/ the same illumination as your surrounding pic, get in close & meter off of it. The reason is you might have a setup where you've got all sorts of variety of color/lighting, etc. The gray card is equivalent to what the meter is trying to read, so it just helps bypass other extraneous info that the meter might be picking up on (a black background, a spot light, whatever).
However, I think that was more a tool for film. Now you can check your histogram, take test shots, or adjust exposure to fit in post-processing, so using a gray card in that manner is not as common. And, your camera has ways to "spot meter" (where it closes in the area it is reading from, really good w/ a backlit shot where you want to meter off the face, and don't want the surrounding light background messing up your meter) and other things that help.