This site already has a question around what is pixel density, but it doesn't mention pixel pitch at all. I am wondering what the relationship is, and what is important about the relationship to photographers today.
Not to foil the answers from others, but this fun equation is what spurred the question:
pixel pitch = 1/sqrt((pixel density per cm^2)/(10000^2))
I have no idea really what the above equation means, or even if it is accurate. I found it at this thread at dpreview.com.