PCD = Pitch Circle Diameter. Pretty much means the bolt pattern of the wheel. 4 lug, 5 lug etc. Most common are 4x114.3 if you are 4 lug and 5x114.3 if you are 5 lug. But there are other ones available such as 5x100 or 5x120 for most German or older Subaru cars. This information will help people look at the right wheel that's for them.
To check PCD they're usually stamped on the inside of the wheel. Just an example,
http://www.savageautosports.com/imag...s/volks006.jpg as a sticker, or even on the rim itself like this -
http://www.savageautosports.com/imag...s/volks007.jpg